I have never seen this feature on any social media platform.

Actually, I bet you have, but it's not really a "mute" function so much as a subscription issue. On reddit, for example, you can subscribe to subreddits. "Muting" them in the case of reddit simply involves unsubscribing, or not subscribing in the first place.
Subscribing/unsubscribing, muting/unmuting, and following/unfollowing topics is currently not an option at all on Steemit. By default, we're subscribed to EVERYTHING without choice aside from the ability to just totally mute a user. To me, that's bad. If I want to avoid any/all posts about sports (I don't, just an example), I don't want to have to mute every user who posts about sports because A) that would incredibly tedious, and B) knowing that most users who do post about sports don't post ONLY about sports, so muting them would be a greater loss than simply muting the sports tag.