There is a fascinating book 'Heirs to Forgotten Kingdoms' by Gerard Russell about minority religions of the Middle East, which includes a chapter on the present-day Samaritans. They claim (although mainstream Jews do not accept this) to be the last remnant of the northern tribes of Israel, their numbers having dwindled over the centuries by defections to other religions like Christianity and Islam.
As Pieter Geerkens said in a comment on your question, there are less than a thousand Samaritans left now, living mainly in one or two villages in the 'Palestinian' West Bank, or in one street in Tel Aviv in Israel. We know from mention in other sources (including the Roman period Jewish historian Josephus) that they were much more numerous in ancient times.
They still have priests and animal sacrifices as per the religion described in the earliest books of the Bible, call themselves Hebrews, not Jews, and will speak the name of their God 'Yahweh' out loud.
Being (they say) northern Israelites they do not accept Jerusalem as a holy city (an innovation of the southern Israelite King David). They reject the later books of what most of us call the Old Testament, which may be innovations of the southern Israelites after their kingdom of Judah separated from Israel or after Judah was exiled to Babylon.
Having long been a small minority existing on sufferance of more numerous and powerful groups in the Middle East the Samaritans have learned to keep their heads down and stay out of politics. However, the description of their practices and e.g. photo of a Samaritan priest with his cheeks ritually smeared with the blood of a sacrificed lamb in Gerard Russell's above book seem to me closer than modern rabbinical Judaism to what the religion described in the Bible from Moses' time would have been like.
Having said that, it may be that after the fearsome Assyrians deported the 10 northern tribes, as the Assyrians probably intended in order to be able to rule them more easily, many of them did lose their Hebrew identity and religion and merge into the then pagan populations of the Middle East. Given the number of times that the Old Testament prophets denounced and punished Israelites for reverting to the paganism of neighbouring peoples, quite a few Israelites must have been fairly ready to do so anyway.
It is therefore quite possible that some of the'Palestinian' and other Arabs of the Middle East who are today mostly such bitter enemies of the modern state of Israel are themselves partly descended from the lost northern tribes of Israel.