Namely adult content. They began heavily moderating this kind of stuff, leading to a mass exodus, in particular to twitter which was amenable to it.
I was drawing a lot at the time, and many of my portraits with no nudity at all were deleted. That was my sign to leave.
I hardly ever saw adult content on Twitter.
When Reddit made its infamous API changes over a year ago that caused many third-party apps to shut down, a less-publicized part of the change was that NSFW content would no longer be available in the API at all, even for apps that paid for the API access.
There was a analysis done a while back which showed the impact that this had on overall posting behavior, because of how Reddit's algorithm works (a drop in visibility across the board will effectively downrank large NSFW subreddits more than small NSFW subreddits, because Reddit artificially boosts lower-volume subreddits in your frontpage so they don't get drowned out). You can also compare total vote counts from August 2023 to June 2023 to gauge the difference too.
Twitter is kind of a cesspool, really. Even more so post Musk.
Even 4chan removes child sexual abuse material at lightening speed. There isn't any major social media site that allows this. If it's even more now, would you be able to name an account that shares such stuff? Or are we supposed to believe this without evidence?
And as far as porn goes sure. The thing about Twitter is you wouldn't see porn unless you specifically go and look for it. Which is why I always find it amusing when people complain about this "problem". There is no porn in people's feeds by default, one has to train the algo to show it by searching for it and interacting with porn accounts. It's especially funny since on Twitter itself there are always conservative or Christian accounts complaining to Musk that they see too much adult content and that he needs to ban it.
I don't know any such accounts because I don't go looking for such things. It's just a well known fact that Twitter has a CSAM problem. A quick google search will show you plenty of media coverage on that fact. It's also well known that moderation on Twitter has suffered under Musk due to massive layoffs.
Of course, most of this porn stuff is never seen by most users because it's not favoured by The Algorithm. I'm just pointing out it's definitely all there. As far as legal porn I'm generally in favour of it being allowed on most platforms.
Also naming names of criminals is not illegal, I'm confident you know this.
As for naming accounts, it's not just "naming criminals". It's effectively equivalent to sharing CSAM if I'm telling people specifically where to find it. Even if I knew where, which I don't, I'd prefer not broadcast that information. And I'm not sure why you're so eager to learn specific account names. It's a pretty weird thing to ask for on a public forum, honestly.
Not everyone has solipsism at the core of their epistemological worldview. Even without knowing your jurisdiction, I would discourage you from seeking out CSAM; the laws do not have an exception for curiosity.
If Twitter didn't have a handle on porn in general, what makes you confident they had CSAM under control?