It hasn't been developed for a while now. Will it eventually just stop working or there's any way to at least port it to qt6, without new features, just keeping the basic functionality?
I have the habit of star most of the songs I listen to, so that it's become easier to, for example, create smart playlists. In the case I can use Cantata no more is it possible to export somehow this data? Let's say that it stops working and I switch to something else (say, Navidrome or some equivalent). Will I be able to export my preferences, stars, playlists, favorites, etc?
Never heard of it, but it will stop working eventually not because of QT, but due to updated distros replacing aging or insecure libraries or compilation tools that will be incompatible with this software. You would have the option of keeping it running on an older LTS release of something, but eventually that will also run out. Best to just find an alternative that is actively developed at some point.