It is very unfortunate, and we put quite a bit of work into seeing if we could get a workaround working before we gave up and accepted that the feature would be broken. For KVM, we actually found a partial workaround (I think you probably saw the thread where I linked to it), but for VirtualBox, it’s just totally broken thanks to Oracle’s code not doing what it’s supposed to.
As a workaround, you could possibly use scripts that will dump the Wayland clipboard contents to a file in the shared folder, then use a script on the host that will copy that to the host’s clipboard? Those could then be bound to keyboard shortcuts that will allow you to do something like “Ctrl+C to copy in guest, Ctrl+Shift+C to dump to shared file, Ctrl+Shift+V in host to load shared file into clipboard, Ctrl+V to paste in host”. How exactly this works will depend on your host OS and display server, but wl-copy and wl-paste will probably be useful on the guest side, and if your host is on X11, xclip will probably let you do the host side of things.
Most of the info I have is from being in the open-source community a lot. A lot of info gets passed around as software develops, and I end up hearing a lot of it since I work with a lot of different developers. (The driver bitrot issue with X11 I’ve experienced first-hand though, as has my workplace.)
Thare are good Linux news sites for hearing about what projects are doing, LWN is one of the better ones. (I don’t have any connection to them, and don’t have an LWN subscription, but I oftentimes read their articles that are old enough to be free.)