mergerfs is, in essence, used to mount multiple drives to the same path, simplifying and automating data-management. There hasn’t been a new release since Fedora 37 and thus no dedicated RPM package exists (mergerfs isn’t available in the Fedora repos).
Can installing a build intended for Fedora 36 (or an older version, if relevant) be considered as safe?
Nothing that is not specifically distributed for F37 can be considered “safe”.
It is quite possible that you can install a package built for a different release version or even a different OS and it may work. Are you aware of the term ‘caveat emptor’ (buyer beware). You assume any and all risks if you modify things from the way it was released and updated, including installing packages from 3rd party sites or different release versions.
That said, I have no experience with mergerfs so cannot give any specific guidance there.