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The survivors are working on it. RedReader is allowed to continue using the API for free because it has accessibility features, but the creator has announced they do not plan to depend on that goodwill and will expand the app to connect to other APIs.

Saidit has a (mostly) compatible API by running a fork from when Reddit was still open source. It isn't a drop-in replacement but most of the core API endpoints are the same. Things like modmail, live threads, collections are exclusive to Reddit, for example.

Not sure the community is welcoming or even interested in the potential growth, tbh. But it exists.

https://saidit.net/dev/api

It would be easy for app developers to quickly port their app to non.io.