Ting I tried that hotkey in Spine 3.8.99 and nothing happened as you mentioned, so apparently there was a bug that prevented this hotkey from working.
It works fine as far as I have tested it in Spine 4.2.35. What this hotkey does, as you might expect, is to toggle the filter on and off in the dopesheet view. Here is a video of this hotkey in working order:

Sorry, but the bugs in 3.8 cannot be fixed, so please consider upgrading your project if you want to use this hotkey.