I have no doubts that Twitter will one day block or crack down on NSFW content and
so do those with ties to the fandom and other Twitter smut producers. A Gizmodo article
compared Twitter's creeping rules against NSFW content to Tumblr's before the eventual ban.
At one time Twitter declared themselves
the free speech wing of the free speech party, now Twitter is known for being one of the most restrictive social networks. The same could very well happen to Twitter and porn if Tumblr is anything to go by.
As for the alternatives, it's a complex situation. FurAffinity is the logical choice when it comes to where many artists will move to, but that site has issues of it's own. Quite a few uploads aren't tagged, some artists jumped ship for social media sites before the porn ban fears were common, the site's code is broken, and the management is poor. Newgrounds has an "old web 1.0 site" reputation and a more insular community and while artists use it I don't see a lot of people going to it. Inkbunny is like the Bitchute of art sites, hosting cub art and dividing potential users because of that. e621 is a booru type site and is popular because of how everything is tagged. It also has a
long and controversial DNP list. Artists end up on the DNP list either to "take a stand" against parent company Bad Dragon, because they got a Kiwi Farms thread, because they got downvoted, or to whine that someone misgendered their character. Pixiv is mainly Asia oriented and
mostly gets views in the West from weebs.
As for non-NSFW and social network type sites, the situation is pretty bad there as well. There's DeviantArt, which disallows porn even with the NSFW option. It's new layout is terrible and traffic has been declining. DeviantArt also is one of the longest running jokes on the internet, being a common place for wannabe trolls and YouTubers to find bad artists to make fun of. ArtStation
also bans NSFW content and tries to orient itself towards professional artists, just like Vimeo. Pillowfort wants to be a Tumblr clone but it charges $5 to register, and doesn't really have people using it. Then there's Mastodon and other fediverse instances (Pleroma comes to mind as the most popular one that's not Mastodon), but those have issues from instances blocking each other to having a userbase that happens to be even worse than Twitter. Good luck getting exposure if some popular instances blocked your instance.
All of these sites have one thing in common. Artists don't get exposure from these sites. Furries using mainstream social media sites happened to be the greatest recruitment tool the fandom had, but once that's gone they're back to where they were years ago. Sure,
apps like PostyBirb help an artist dump their content to dozens of art sites at once but does that mean much when nobody is going to these other sites? Even if someone starts up their own DeviantArt clone, setting up a site like that requires the owner have multiple skills including programming knowledge, how to run a site like that, and how to attract users. It's telling that the only non-FurAffinity furry art site that's popular is one that caters to a very specific demographic kicked out of FurAffinity.