Do you know if it’s still possible to add a shortcut for the brush “clear” option? My PS version is 27.4.0 and I can’t get it to work with either the ~ key or the Alt+Shift+R shortcut.

Yes i tested it on both Mac Tahoe 26.4 and Windows 11 with Photoshop 2026 (ps ver 24.7)

and after with the 24.5 update and both the tilde key and Alt+Shift+R (Option Shift R) seem to work.

There two ways that I can think of, but they are deeply flawed. 

You can use the , and . (coma and full stop) keys to select previous and next brush preset, but AFAIK this only works with the selected brush group, and not the Recent Brushes presets.  I’m thinking you really need it to work with whatever preset you happen to be using though.

The second is hardware dependent, and needs a Gaming keyboard.  If you have such a keyboard, then using its set up (iCue, GHub, Synapse etc.) then you can apply the Shift Alt R, and Shift Alt N key strokes to either a dedicated G key, or any other key with some keyboards like the Logitech G515.  Or you can use an extension keyboard like the Razer Tartarus

Or a smaller keypad.  Unfortunately, and AFAIK, it is not possible to use an Action to record the above shortcut, but maybe a script.  Or we could just find out why the Tilde key is now working for us. :-(

A reset of the Photoshop preferences might fix the issue:

https://helpx.adobe.com/photoshop/desktop/get-started/settings-and-preferences/reset-preferences.html

There is also the just released 27.5 update you could try before a Photoshop preferences reset.

In Photoshop go to Help>Updates