Found it on Reddit. It’s most probably incomplete, but it has a “Suggest link” button.

The rules for submitting a link are (as of recent):

• No links to communities who break their service’s ToS or share any illegal content (this should be self-explanatory).
• Link must be an alternative to an existing subreddit. I don’t see sub.rehab as a general fediverse directory (for now).
• Community must have a minimal amount of activity. I am thinking - at least 3 posts that are not submitted by the community’s moderator team. This does not apply for communities that are verified and proven as official replacements of the original subreddit