Hi, 
I just plug my voice here for **a big +1 to this**.
(even if the bug is in "CONFIRMED" state, wich is great). 

Not having this possibility on Wayland is a big regression compare to kcm_tablet on X11.

For digital painting, modifiers like "Alt", "Ctrl" or "Shift" are **essential**.
I'm using on my side to have "Ctrl" modifier on the first button of my stylus (to replace the default "middle click/drag/scroll" action on GNU/Linux).
"Ctrl" allows me to color-pick canvas quickly. This is something I use since I started digital painting before 2000, so the muscle memory for that is strong on my side. 

( A short story: I remember the Gnome GUI not allowing this before 2013. On my side, I was praising already more than 10 years ago the configuration possibilities of KDE/Plasma GUI https://www.davidrevoy.com/article110/kubuntu-11-10-for-digital-painting , and during my digital painting workshop at the Libre Graphic Meeting in Madrid two Gnome designers asked me why I was using KDE/Plasma; then they realised when I showed the kcm_tablet GUI.  After this session, it was fixed the same year on GNOME side. The story is here to say it is ironic we are now ten years later, and it is Gnome who has the options for the tablet and no more KDE/Plasma and we have to re-explain how important it is ... )

A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/4797

A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/1933

Git commit adbf69aab49a193650d895bbcee11899a968f17d by Joshua Goins, on behalf of Tino Lorenz.
Committed on 20/01/2024 at 16:49.
Pushed by redstrate into branch 'master'.

plugins/buttonrebinds: Support rebinding to modifier keys

Allow rebinding buttons to send modifier keys (Control, Alt, Meta,
Shift) without any other key.
This is part of restoring the functionality kcm-wacomtablet had under
X11 for wayland.

M  +23   -4    src/plugins/buttonrebinds/buttonrebindsfilter.cpp

https://invent.kde.org/plasma/kwin/-/commit/adbf69aab49a193650d895bbcee11899a968f17d

A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/5008

Git commit 3066dbdb8d8a0668f805482fb333882e952e08bf by Joshua Goins, on behalf of Tino Lorenz.
Committed on 20/01/2024 at 17:23.
Pushed by redstrate into branch 'master'.

kcm-tablet: Allow binding buttons to modifier keys

Allow rebinding buttons to send modifier keys (Control, Alt, Meta,
Shift) without any other key.

M  +2    -0    kcms/tablet/ui/main.qml

https://invent.kde.org/plasma/plasma-desktop/-/commit/3066dbdb8d8a0668f805482fb333882e952e08bf

A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/5009

Git commit 5a0f716236b812b9742a987bd260d98aa9c2e350 by Vlad Zahorodnii, on behalf of Tino Lorenz.
Committed on 22/01/2024 at 09:36.
Pushed by vladz into branch 'Plasma/6.0'.

plugins/buttonrebinds: Support rebinding to modifier keys

Allow rebinding buttons to send modifier keys (Control, Alt, Meta,
Shift) without any other key.
This is part of restoring the functionality kcm-wacomtablet had under
X11 for wayland.

(cherry picked from commit adbf69aab49a193650d895bbcee11899a968f17d)

M  +23   -4    src/plugins/buttonrebinds/buttonrebindsfilter.cpp

https://invent.kde.org/plasma/kwin/-/commit/5a0f716236b812b9742a987bd260d98aa9c2e350

Git commit c76724b64e3a4962b22890aac54a123c5002c046 by Joshua Goins, on behalf of Tino Lorenz.
Committed on 30/01/2024 at 21:34.
Pushed by redstrate into branch 'Plasma/6.0'.

kcm-tablet: Allow binding buttons to modifier keys

Allow rebinding buttons to send modifier keys (Control, Alt, Meta,
Shift) without any other key.

(cherry picked from commit 3066dbdb8d8a0668f805482fb333882e952e08bf)

M  +2    -0    kcms/tablet/ui/main.qml

https://invent.kde.org/plasma/plasma-desktop/-/commit/c76724b64e3a4962b22890aac54a123c5002c046

I am currently testing Plasma 6.1 Beta and I can assign Meta as a shortcut as expected in the global shortcut settings, but if I try to set the shortcut in separate configuration windows (e.g. by going to Desktop Effects -> Overview -> Configure) it is not possible to set modifiers by themselves there.

@adrian.groh : I'm not sure to understand how your observation is related to the 'Graphic Tablet panel' or to physical button of Graphic Tablets. It sounds more like a general feedback about Plasma and shortcut (global shortcut settings and Desktop Effect). 
Can you clarify?

(In reply to David REVOY from comment #11)
> @adrian.groh : I'm not sure to understand how your observation is related to
> the 'Graphic Tablet panel' or to physical button of Graphic Tablets. It
> sounds more like a general feedback about Plasma and shortcut (global
> shortcut settings and Desktop Effect). 
> Can you clarify?

Oh sorry, I mistakenly thought this bug was about being able to assign modifier keys alone to any action. Please ignore my previous comment.

No problem. Thank you for the update, and I hope you'll find a better thread (or open a new one) to post your observations. Good luck!