> Say in a toolkit the text input widgets don't support pasting content.

That's a bug.

> various UI frameworks have long had issues with different paste board systems on Linux

In my experience on *nix (37 years, going back to SunOS), the paste board systems for X Window are very well defined, and the only UI frameworks that have "long had issues" with them are buggy and not widely used. Are you thinking of something in particular?

Pasting between Xlib, GNOME, and Qt apps was a shit show for years. The situation has improved significantly since the advent of Freedesktop.org but there used to be lots of problems. It doesn't matter if a UX problem is a bug or design problem. A user doesn't control if some app decides to use some unpopular UI framework. It's a Linux-on-the-desktop problem no matter the ultimate cause of the issue.

There's still issues today on Linux with keyboard shortcuts. Some apps use Ctrl+Shift and others use only Ctrl, there's no way to know what an app uses a priori. You have to hope the shortcut is displayed in the menu and you remember it when you need to actually use it. Different frameworks have different default shortcuts for common behaviors. Accessibility for any individual app is still hit or miss, same with localization and theming. Mac and Windows users don't face anywhere near the same impedance mismatch between various first and third party applications. Those systems are far from perfect but they're usually much more consistent UX than a Linux desktop.

For the sort of "creative" apps I'm interested in, kbd short cuts other than Ctrl/Cmd-[cxv] are the domain of the app, and more or less every possible combination is used. My own app has more than 400 shortcuts just when installed with no additions by the user. What the OS or other apps do is of no relevance for creative apps (except for other similar apps).