I’m guessing the admin stopped caring as much about the mission of the site, which actually seems quite compelling, or it wasn’t as profitable as they thought, and was looking for a reason to divest themselves of it. Spam is hard to fight, it does suck in that manner, but certainly possible and certainly pulling the plug on an otherwise passion project screams “don’t care anymore.”
Personally I didn't wanted to invite to more guesses, however if this was because of any of those reasons, it does not look to me there was anyone in a hurry:
txti @txties 15. Juni 2020: "I realize txti is broken at the moment, and I'm going to try to fix it. Truth is, I haven't really been doing web development for a few years and projects like txti have unfortunately taken a back seat to life."