Looks like Valve did a bit of a woopsie. With the recent updates to Counter-Strike: Global Offensive they implemented a new anti-cheat tool with Trusted Mode but it appears at some point they forgot to enable it.
What is Trusted Mode? It's supposed to be the new default for all CS:GO players, which prevents a bunch of outside applications from interfering with it and hopefully prevent more cheating. It's only a small barrier by itself, just another in the list of ways Valve are trying to clean up CS:GO online play.
At least in the Linux version, it's actually not on by default (bug report - verified here at GOL too). If it's off, Valve noted in CS:GO updates that it may cause your Trust Score to be "negatively affected" so you might see more cheaters and terrible people. If you have the developer console enabled, you can run "trusted_launch_info" and it will tell you if it's on or off.
Thankfully, it's a super simple fix. Add this as a launch option until Valve sort it:
So if you plan on settling into some CS:GO this weekend, this quick fix should help and you can carry on gaming on Linux.
Play CS:GO free on Steam.