The itch.io version is an "unsupported platform" and you can't create or use an account here. That locks out a lot of game content (including the ability to use a name of your own choice, and free chat) so functionally you only get the Steam version.
Thanks to Valve turning the Steam client into a health hazard almost 2 years ago (and blocking access to any safe workarounds since), I was forced to refund the game on itch.io because I don't have the full game here. I was able to get the game on Epic (as bad as they are, still better than Valve at this point) so I can still play.
it's because of at least half a dozen different problems, all of which showed up in the "new library" update in 2019, and most of which are long-standing known problems for numerous medical conditions which everyone (except apparently some idiot at Valve) knows better than to include in a game launcher app.