So, dumb question incoming:

How hard would it be to make a thing with the same API as Reddit so the apps that are currently about to shut down can instead change base domain and keep going?

Including existing OSS servers, assuming there already is some (complete!) open source one somewhere — I can google for the attempts ("plebbit" was submitted here for discussion only last week) but I can't trivially tell if they're any good.

Probably not hard to write an adapter at all. But it's not the API that made these apps popular and appealing. It's the Reddit community. The apps would write their own adapters if there was an alternative social media website with this community.

If you flipped the switch on the apps, you'd have your community in no time

Iirc such things already exist.