There's internet culture precedent for this. 4chan itself has an early history of picking up leftovers from communities that were banned on other sites/forums/platforms.
4chan's origin itself fits that archetype, as well. It was created when a hentai subforum got banned on a larger forum, and the community moved over to the new imageboard.
This already happens, and I don't see how a law like this would significantly change the volume of edgelords and incels funneling into imageboards