Is there a way to revert manually to earlier versions of a package?

If you are using TeXLive, tlmgr can backup installed versions (it already does this automagically) and restore previously installed; see its documentation. I don't know if MikTeX offers something similar. And the mercurial repository of CTAN offers older versions - but you'll have to install them manually.

If you use tlmgr, you can run tlmgr restore [pkg] to find what backups exists. If you have a version you want to restore to, run tlmgr restore [pkg] [revision number] to actually do the restoration.

Historical versions of (some) packages were kept on an Automated Mercurial Repositories of CTAN which has since been taken down. The most recent available version is available on Wayback Machine - The Internet Archive (dated Oct 5, 2016). However, you'll have to manually install these in your local /texmf folder.

A repository of CTAN package versions would be very useful but it appears this 'Automated Mercurial Repositories' is no longer available.

Note: Automated Mercurial Repositories host http://ctanhg.scharrer-online.de no longer appears to be registered.