This site lists the communities on -shall not be named- that have either migrated or are also located on different sites. It also lists whether it is the official community or a spin off. A very helpful tool for those who have rid of -shall not be named- and have forgotten communities they once followed.
If you know of a community somewhere in the Fediverse that serves a similar purpose or is an official replacement for a subreddit, you can and should post it as a suggestion to help people migrate!