As far as I can tell there is no way to prevent a user from muting a category. I use the “Site Feedback” category for discussions with users, and for announcements. I also have a separate read only category for User Guides.
Many of our users are only interested in some categories, and only use the email functionality, so they mute everything else. This is fine until I make an announcement, or explain some feature in one of the two categories above. If the user has those categories muted then they won’t see that info.
Some way of making a category unmutable would be useful.