is the [patch](https://gitlab.gnome.org/GNOME/gtk/-/issues/5397) to Scintilla needed for Scintilla to actually provide that format
I'm not certain but the test in the Scintilla code is against the local small integer (enum) `TARGET_UTF8_STRING` and GTK is most likely converting both `UTF8_STRING` and `text/plain;charset=utf-8` to `TARGET_UTF8_STRING` before calling back into Scintilla thus causing the same action for either request.
Without documentation of the correct way to implement clipboard types, I see lots of potential for causing new problems. Compatibility seems to be getting worse with updates to applications, toolkits, and window systems. Maybe different target lists should be provided on X, Wayland, Win32 and macOS.
If the Geany project tries the patch out on your supported platforms and are happy with it then it can go in.