I recently released Thaw, a command line tool for upgrading flake inputs based on semantic versions. Thaw works by querying an input’s associated forge for tags and upgrading the version based on SemVer rules! No server or additional software is needed, just git tags!
Currently the only forge implemented is GitHub, but Thaw is easily extendable to support more flake ref protocols and I would be happy to review any PRs from the community.