The mute on the touchbar on my MacBook Pro has a slash when unmuted (and also when muted).

It looks like this unmuted: https://discussions.apple.com/content/attachment/e909bffc-81...

and this muted: https://discussions.apple.com/content/attachment/e3e9c230-e4...

I believe the reason it always has the slash is to distinguish it from the "change volume" button (which, when pressed, pops up a volume slider). But it's still confusing.