There's already dozens of forks. There was before all of this even started. Sad to see it go, but the app itself and how it functions will pretty well certainly go on. Tachiyomi isn't even an app that needs routine updates, it's the extensions that do. So existing setups will probably work indefinitely so long as extensions continue getting updates...

So, sounds very "oh fuck" but for end users probably means literally nothing changes. Unless something breaks the functionality of the app, which really just means the reader and library and everything, it doesn't need active maintenance. Bugs might crop up occasionally, but everything looks fine as of now.