EDIT: Lol, it doesn’t actually work +___+ It is enabled in KeepassXC but it just doesn’t do anything. Welp.
Here’s a neat trick I just found out (with a hint from here):
In Wayland you can’t use KeepassXC’s very cool Auto-Type feature (it’s somehow Qt’s fault?) but if you installed it as a Flatpak you can go into KDE Settings, search for “Flatpak Permission Settings” and in the settings for KeepassXC under “Advanced” you can disable “Wayland Windowing System” to make it work. Nice!
Browser integration works on my machine, which also uses Wayland, so unless you're, say, running Firefox from a flatpak or something, I don't see why it shouldn't work.