Me and my friends set up a sever to play Attack of the B-Team on, when we logged into the server we notices none of our skins were showing, only default Steves. We then decided just to play the latest version which did allow our skins to show up. I say allow because I tested it on other versions, It's not the servers that are the problem, it's the version that the game is being played on. I know next to nothing about coding games but I know for a fact that something went wrong in this last update (1.12.2) that made it so only on and above to be shown.

Minecraft isn't unplayable without a skin but imagine you grow attached or maybe you spent quite alot of time on a skin, you're going to be a little upset if the skin you made or you like stops working, especially if it's something you yourself can't fix.