It is yes, they've even built a command for it "rebuild"[0]
> Reconstruct the named repository database from the core records. Run this command after updating the fossil executable in a way that changes the database schema.
Afaik fossil just uses sqlite under the hood to store metadata. It would surprise me sqlite doesnt stash min and max required versions and refuse to work outside of those bounds
It does do exactly that. If you visit https://sqlite.org/src/stat, the "Schema Version" line tells you exactly what version of the database schema that the repository is using. The SQLite repository uses the very latest Fossil schema, which you can see from the "stat" page has not changed in 8.5 years.
> Reconstruct the named repository database from the core records. Run this command after updating the fossil executable in a way that changes the database schema.
[0]: https://fossil-scm.org/home/help?cmd=rebuild