This piqued my curiosity, and it turns out that Þ is an obsolete Old and Middle English letter, which was pronounced almost like th.

There’s a subreddit called BringBackThorn, with the description below. I wouldn’t be surprised if there’s a similar trend on the Fediverse.

Bring Back Þ is a subreddit for þe movement to reintroduce þe letter Þ, pronounced “thorn,” an Old English letter representing þe ⟨th⟩ sound þat fell out of use wiþ þe advent of þe printing press. Printing press tilesets imported from Europe didn’t have Þ as þeir languages lacked þis letter and several oþer old English glyphs.

TL;DR: Let the dude be, he’s not harming anyone with his hobby.