Ignoring the political issues, my problem with Twitter is more prosaic: the character limit is too short for basically anything of substance. I really don't like the thread style it effectively forces.
(Oh, and the new name is unbelievably stupid. I ain't never calling it that.)
This works both ways for what it's worth, I have parts of my identity which I simply don't see fit to make a discussion of since there is no ground to gain or concede.
The other thing is the sarcasm and smugness.
The decline happened with bots flooding the platform, with the "creator reward" program rewarding meaningless engagement bait, and with people who I respect in the Tech bubble leaving the platform, which led to an even faster decline.
That was just not the case before the takeover; back then, you would enjoy scrolling through it, engage in authentic conversations, and learn a thing or two.