Fyi “I prefer my doctors clean shaven” is my Johnlock obsession of the day
All the past incarnations of Watson have mustaches and ours is marked by his clean-shavenness as something new and modern. John only has one when he’s ready to marry Mary, which he thinks is “moving on” but the mustache is actually a visual step backwards (Mrs. Hudson tells him twice that ”It ages you”). It’s a symbol of stepping into the traditional Watson role pre-BBC Sherlock
Then Sherlock comes back and it’s gone again, because it was “Just something I’m trying out.” The traditional Watson role doesn’t suit our John, and Sherlock agrees: he prefers his doctor’s clean-shaven