So I've never really posted here before (lurker=me), but here are my two cents for whatever it's worth.
I don't use Facebook - never have, never will. So if the forums shut down, that would be it for me. I truly hope it doesn't come to this since come the new year, I should have more time to commit to writing (helping to open a new business has taxed my sanity, wallet and free time). I haven't use Discord, either, yet I'd be open to that.
I have in the past casually mentioned/linked Worldsmyths a few times at the place where I've been more active. It's a mostly Harry Potter fanfic forum+archive (eFiction - no bridge between sites/multiple logins), but we have members venturing outside of fanfic and getting serious about writing original fic. Let me get with TPTB (the powers that be) over there about how they'd handle affiliation.
tl;dr - please keep forums open!