What a wonderful statement.

It's actually surprising that this is not configurable already. At least in a GUI.

I guess one could create shortcuts to a tool like wl-copy and wl-paste to either copy or paste content to primary selection (or regular clipboard for that matter). So in that case a simple script could run the command and in your desktop environment you setup a shortcut to run the script.

Yes its hacky, but in Linux nothing is impossible. :-) (unless it is...)

Then I tell you something that might either blow your mind or be useful in future (or just being fun fact):

On Linux there is the regular copy/paste clipboard, which you already know how it works. But then there is this primary clipboard called primary selection too, that is independent from normal clipboard. Text will be copied to primary selection when you select a text (in example in Firefox). Just by selection the text with the mouse is enough and it will not affect the normal clipboard. Then you can middle click the text from primary clipboard.

Read more here: https://tronche.com/gui/x/icccm/sec-2.html#s-2.6.1

They only discuss to disable it by default, not removing the functionality.

The essence of the article:

The discussions, visible in Mozilla’s Phabricator revision D277804 and a linked GNOME gsettings-desktop-schemas merge request, focus on disabling the traditional primary selection paste by default.

Mozilla proposes changing the default behavior of the Firefox browser on Unix builds so that pressing the middle mouse button no longer pastes text by default.

The functionality will be there and can be enabled. The reasoning:

The author of the revision frames the current behavior as a source of confusion and accidental pastes, especially when users press the middle button without expecting the clipboard contents to be inserted into text fields.

Saying that to an indie studio is mean.

Well, that's one thing with Flatpak. There is a permission system, as the applications are fully or partially sandboxed. You can install "Flatseal", that can change permission for each installed Flatpak application. But it can be confusing or hard to understand what you have to change in order to make it work. Or maybe the application itself is not packaged correctly as a Flatpak, I don't know.