I know someone[0] who is working on exactly this right now: https://fossilrepo.io/

I don't think he's got public sign ups turned on yet. Maybe hit him up on the Twitter for more info.

[0] - https://x.com/ragelink

There's also "Chisel - Fossil SCM Hosting".

"This service is completely free and run because a service like it should exist."

https://chiselapp.com/

"All public repositories":

https://chiselapp.com/repositories/

I don't know if it has all the GitHub features people may be looking for.

Chisel runs on Flint, "The ISC licensed codebase behind http://chiselapp.com.":

https://chiselapp.com/user/rkeene/repository/flint/index

EDIT: Add "...should exist." sentence from the homepage.