I think AT&T in 1982 was the last company actually broken up. It seems like the DOJ tries to keep companies from getting too big in the first place now, with mixed results. In recent times Penguin/Random House/Simon & Schuster was blocked, AT&T/T-Mobile was blocked, but Microsoft/Activision succeeded

https://en.m.wikipedia.org/wiki/Breakup_of_the_Bell_System

https://www.cnbc.com/2022/11/01/judge-blocks-penguin-random-...

https://en.m.wikipedia.org/wiki/Attempted_purchase_of_T-Mobi...