There is a new wayland extension, that allows for rendering at fractional scales directly in the application, so when you use a display with fractional scaling, there is no downscaling done by the compositor.

Support for this kind of rendering is going to wait until Gtk 5 -- implementing it in Gtk 4 would break ABI, and then the world would have to listen to cries about Yet Another Breakage. The current system, rendering at integer scale and then compositor-downscale is working fine since Gtk 3. Btw, exactly this way is how Apple does it, and it was lauded as a great way to support scaling.

I think "mostly looks fine" is the crux here.

I also believe this is only the case for Wayland, so X11 users may be out of luck here. Actual fractional support would require breaking API compatibility, which is why integer scaling+downscaling is necessary.