>>googlr+cm
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