If there was any reward at all for the helots at Plataea, it was plunder. There is no evidence from ancient sources that any helots were made citizens in the aftermath of Plataea. Also, there is very strong circumstantial evidence which suggests that citizenship was not awarded to helots. The earliest likely date for helots becoming citizens is the late 3rd century, at least 250 years after the Battle of Plataea.
There is also no evidence that any helots were freed in the aftermath of the Battle of Plataea, but it is not inconceivable that a small number were. This raises the question as to why a servile population apparently willingly supported their masters; this issue is discussed at the end of this answer.
The role played by the helots at the Battle of Plataea (479 BC) - and how many of them there were - is disputed. The only source is Herodotus who says there were 7 helots for each of the 5,000 Spartiates (citizens):
this would certainly be the
largest number of Helots ever known to have left Lakonia. In fact to
many scholars it has seemed implausibly high. Clearly it was not
demanded on strictly military grounds, although Welwei (1974, 120–4)
has properly stressed the supply problem of this campaign and
suggested that Helots were used to solve it.
Source: Paul Cartledge, Sparta and Lakonia
Thus, many helots may not even have fought but, for those that did, there is no evidence in ancient sources that any were made citizens.
Despite the Spartan citizen population declining steadily over the years from around the late to mid 5th century BC, only those born to a Spartiate father and a Spartiate mother could be citizens. Even the sons of Spartan citizens with helot women (mothakes) could not become citizens.
Also at Plataea were 5,000 perioeci hoplites. These were free non-citizens who had no political say in Sparta and were obligated to provide soldiers when called upon. They were of a higher social class than the helots, and yet there is no evidence that any of them were made citizens for the military service they rendered at Plataea. If the perioeci and mothakes could not gain Spartan citizenship, it is highly improbable that helots could.
Although the shortage of citizens became increasingly critical during the 4th and 3rd centuries, proposals to increase the citizen body by granting citizenship to some of those outside the elite were rejected until Cleomenes III (235 - 222 BC) forced through much needed reforms after an earlier King, Agis IV (245 - 241 BC), had failed.
We cannot rule out the possibility that some helots were freed in the aftermath of Plataea, but there is no mention of this happening in ancient sources and it does not appear to have been the practice at the time of Plataea. If any were freed, they would most likely have become perioeci and been given land on the frontiers of Lakonia or other territory controlled by Sparta.
The practice of freeing helots for military service rendered does not seem to have happened until around the 420s (see Neodamodes, and also those who fought with Brasidas during the Peloponnesian War). The JSTOR article On Messenian and Laconian Helots in the Fifth Century B.C. goes into some detail on this by examing the ancient sources.
This change in 'policy' was likely due to the apparently heavy losses Sparta suffered in the earthquake of 464 BC, and the casualties suffered during the Peloponnesian War. Note how concerned the Spartans were at the possibility of losing just 120 citizens captured after the Battle of Sphacteria (425 BC). Freeing helots was one way of tackling the problem of declining manpower.
Why did the helots support the Spartans at Plataea?
So why did the Helots apparently willingly support and probably fight alongside their Spartan masters? There is no definite answer to this question, but the most plausible explanation is that the Lakonian helots (who almost certainly comprised the bulk of the helots at Plataea), perceived that they had little to gain and much to lose by swapping their Spartan masters for the powerful Persian empire.
We should also not ignore that the Lakonian helots seem to have had a not entirely disadvantageous relationship with the Spartans. They were not like most slaves who could be (and often were) separated from their families, and they had land to work from which they could profit to some extent - even if they didn't own it, they had ancestral ties to it. Thus, if they had turned on their Spartan masters, they would have had much to lose, and the other Greek states in the anti-Persian coalition would certainly not have thanked them for it (i.e. it wasn't just the Spartans they were helping, it was most of Greece). As Peter Hunt puts it,
Many cities, such as Athens and Aegina, had deferred their bitter
quarrels to fight together against the Mede. The Helots too may have
felt that their fight with the Spartans could be put aside until the
Persians were repelled. They also would have known that, although they
outnumbered the Spartiates, they were but a part of a large Greek
alliance under Spartan leadership and that their homes and families
were under Spartan control.
Source: Peter Hunt, 'Helots at the Battle of Plataea' (Historia: Zeitschrift für Alte Geschichte
Bd. 46, H. 2 (2nd Qtr., 1997)
The Spartans may have been the helots' unwanted masters, but at least they were Greeks, not 'barabrians' like the Persians. Nor was helot support in battle unprecedented; they were among the dead at Thermopylae and were buried next to the Spartans. Also, according to two ancient sources, Lakonian helots were used during the First Messenian war, and there is other evidence of the trust placed in them by Spartans. Thus, it is important to differentiate between Lakonian helots and Messenian helots:
There is simply no indication of trouble among the Laconian helots in
the 490s. Indeed, if the Spartans did actually face a Messenian revolt
at this time, their ability to contain it and then immediately send a
large hoplite force to Marathon almost reqires the loyalty of their
Laconian subjects. Any unrest in the 490s, therefore, involved only
the Messenian helots....
...Given a history of helot rebellions in Messenia, it is logical that the
Spartiates were wary of trouble from that quarter. On the other hand,
since there is no corresponding evidence of unrest in Laconia, it is
equally reasonable that the Spartiates trusted them enough to rely
upon them as support troops.
Source: James T. Chambers, 'On Messenian and Laconian Helots in the 5th Century BC' (The Historian, Feb 1978)
It has been argued by Friedrich Cornelius, in Pausanias (Historia: Zeitschrift für Alte Geschichte Bd. 22, H. 3 (3rd Qtr., 1973)), that Pausanius promised some helots freedom after Plataea, but this is speculative; there is no evidence that Pausanias conspired with the helots before Plataea. What Thucydides says clearly relates to not just after the Battle of Plataea (479 BC) but also after Pausanias had been accused of conspiring with the Persians (478 BC) (see also Thucydides) and adopting some of their habits. According to Thucydides, the Ephors (five annually elected 'magistrates')
were also informed that he was intriguing with the Helots; and this
was true, for he had promised them emancipation and citizenship if
they would join him in an insurrection and help to carry out his whole
Note that the promise of emancipation was for joining an insurrection, not for services rendered at Plataea. A more likely 'direct' inducement (if there was one at all) was plunder:
After the battle of Plataea Pausanias ordered the helots to collect
the booty left behind by the Persians. The booty then became public
property and anyone found in private possession of it could be put to
death. The helots stole as much of it as they could hide, and then
sold the stolen goods to the Aeginetans. Herodotus probably
exaggerates when he says that the Aeginetans founded their national
treasure with the gold that they bought from the helots ; still the
plunder stolen must have been considerable. It is inconceivable that
theft on such a grand scale and the sale of the spoils to another
state should have remained a secret. Yet Pausanias apparently did
nothing to recover the booty or to punish the helots.
Source: Borimir Jordan, 'The Cermony of the Helots in Thucydides' (L'Antiquité Classique T. 59 (1990)
Hunt makes the same point:
The story that the Helots stole booty from the Spartiates may well
conceal the first rewards they reaped for their service; usually, the
Spartans exercised close supervision over the distribution of captured
Based on the available evidence, it is most likely that the vast majority of the helots simply returned to their former lives (and families, as well as the land to which they were tied) after the battle, but that some may have benefited from plunder.
Anton Powell (ed), A Companion to Sparta
Nabis of Sparta and the Helots