Are there examples of technologies developed since 1984 that would not have been likely to be developed had the Bell system stayed intact rather than being broken up by the US government?
AT&T wasn't very successful at developing technology for the marketplace in its final decades as a federally legislated monopoly. They were world class in developing innovative technology, the envy of the world, which sat on a shelf almost devoid of use because AT&T operated outside of the free market and had never developed the need or instincts to introduce such technologies which their world class research facilities produced.
AT&T was a federally mandated monopoly which made wheelbarrows full of money charging exorbitant prices which were independent of market forces and competition. AT&T was used to charging 2-4-10$ a minute for long distance calls which could be as short as in an adjoining state. In a time when technology was doubling the efficiency of fiber optic lines every year, dropping the need for costly new fiber lines, AT&T continued to raise prices. After the break-up, new companies like MCI's business model was to lease existing circuits from AT&T 95% of which were dark from being unused and offer long distance service at $0.02 a minute for which days earlier AT&T was charging $4.00.
AT&T had no market pressure nor need to innovate, take risks, or promote technology which would allow them or others to be more efficient and responsive to market forces.
Given this, AT&T for decades had the money to plow into first class research facilities like Bell Labs which pioneered many important discoveries in computing, computer science and telecommunications. But under AT&T most of these discoveries lay inert, unable to fulfill their potential because AT&T did not see any need to roll them out as independent technologies which the market could consume.
Unix(1971), C(1972), C++(early 1979).
AT&T developed Unix in 1971, and it, and later its variants was widely regarded as the gold standard of operating systems. But AT&T never introduced it commercially into the market at a price point most people could entertain. AT&T spent their time suing people who tried to market it independently. It wasn't until decades later when others (SCO, Berkeley, AUX, and finally Linux) recreated AT&T's codebase and introduced Unix's concepts independent of AT&T that the technology tapped its potential. AT&T's greatly impeded the technology from being introduced on the market for decades.
The same can be said of C which took 15 years to really penetrate the commercial market and then not due to AT&T, but due to companies like Borland, Microsoft and IBM which marketed their own compilers. Even then there was no AT&T competitor on the most popular computing platforms for C or C++.