> suffer whenever either of the sources goes unreachable

This is not the way Palomar works. From the About page (https://palomar-registry.org/about):

> Palomar does keep a public preservation fork of every registered source, solely as a backup for the registry in the event that the original repository disappears.

The decision to limit git sources to Github is likely in order to be able to use Github's fork mechanism. Palomar could still offer to take a copy of the relevant commit of non-Github repositories.