Quoting from the proposal:
Qt Wayland plugin has been available for a long time, but it
hasn't
been in condition where it could be enabled by default. With Qt 5.12
the state of the Wayland plugin is much better and it's becoming more
and more reliable. It now supports all the needed protocols
Actually, the primary selection protocol (middle-click paste) is only
supported in Qt ≥ 5.14 (not released yet). (It also requires a compositor
implementing the final specification. I'm not sure whether GNOME Shell
already moved from the GNOME private version to the upstreamed version of
the protocol.)
Kevin Kofler