Twitter, like just about every site, is reacting to multiple things at once rather than just SESTA/FOSTA, but in general they are doing multiple things. Following are the things I have observed, but I am sure that others can add to this list.

They have changed their ToS to say that nudity/porn is not allowed (although there is still a ton of pictures/videos being posted). According to the policy, they may suspend/ban an account for this type of activity.

They are "shadowbanning" accounts, which eliminates the account's posts coming up in searches or in anyone's feeds other than those who specifically follow that account. Nobody knows exactly what creates a shadowban (I was shadowbanned at one point because I had DM'd several Dommes about travel things, but hadn't gotten any responses) or how long one lasts, but I have seen speculation that if you use adult oriented tags, post adult oriented pictures, retweet adult material (especially from shadowbanned accounts) or exhibit behaviors that make you look like a spammer (such as what I did with the DMs) then you are much more likely to receive a shadowban. It is not something that is announced & the only way to really know if it the case is to make a post with a specific tag, then log out of Twitter & search for that tag a little later on (tags do have to be indexed so it may not show up immediately anyways).

As for SESTA/FOSTA itself, I am sure that if you publish hourly rates for a specific activity you are on the track to get suspended. I don't know if this is true within DMs as well or if that is just related to public posts, but it is probably better to use a secure email account (such as one from Protonmail.ch or Hushmail.com) or the phone for those types of discussions.