Modern distributions no longer have the deprecated library in repos.

Not true. libgnome-keyring is still available in Fedora (even Rawhide), even now (8 months after this commit). Moreover, if the library is not available, the plugin will simply not be built, so I do not see the point of removing it.

I would suggest reverting this commit until you have a replacement plugin using libsecret (or an interoperable Qt/KDE library if that finally happens).