In 3.1.0-beta3 gtk4-client will still have "in development" status, but that's supposed to change to "supported" in beta4 (last beta)
Relative to 3.0, Qt-client's update to use Qt6 is the major feature. It's still buildable also in "Qt5-mode", and that's what Windows < 10 builds do, as Qt6 requires Win10.
gtk3.22-client is still the default client.
sdl2-client has improved significantly since early days, and it is the best client, e.g., on tiny screens. Also, I've found that it's the only usable client on touchscreen devices (and even it is not great)