It's been highlighted in T88115 that in sculpt mode, the Shift+S key has been mapped to both smooth brush and stabilize stroke. Only smooth brush works, likely because of event consumption which, realistically, is a good idea. If we really need to...

...apply more than one operation, we probably don't want it done in parallel and can write a Python script for that. So, I've changed the default key for smooth brush to Alt+S.

The exact key is clearly open for change, but Alt+S is syntactically close enough to "Smooth Brush", does not interfere with other operations, and works in testing. The only real limitation I'm feeling is that the user now has to press 'Alt' to switch brush modes, but this should be small enough to be inconsequential.