Update: here's a fresh Steam Deck guide with Prism Launcher.
Unfortunately, it seems a developer on the Minecraft launcher PolyMC went completely rogue and so for your own safety, you should remove PolyMC from all systems. Last night I was made aware of the situation and posted about it on Twitter but for those who don't follow along, here's what happened.
14 hours ago, according to GitHub, the creator removed the Code of Conduct with a commit message titled "reclaim polymc from the leftoids". After that, they then kicked out the other developers from the project in some kind of completely hostile takeover.
People thought they got hacked but unfortunately this person seemingly just decided to attack others. In a message in the PolyMC Discord, they said this:
Regardless on your views, this kind of behaviour is not something anyone of any sane mind should support. What they've done is only show how they are completely untrustworthy, and they've basically killed the project. You should 100% consider PolyMC to be compromised and move onto another launcher. There's nothing to indicate malware or anything like that but any trust is simply gone, who knows what they would do next?
Sadly, this means the various videos and guides I did on PolyMC are no longer valid, and so I will be looking to cover other launchers in future. You should also revoke any permissions you gave to PolyMC via your Microsoft account.
What are your options for right now? You can try:
Although, MultiMC comes with its own controversy for Linux packaging and licensing stuff but at least the developer isn't malicious.
Is there some good news? Seemingly so. The team who were kicked out have banded together to carry it on under PrismLauncher but it's very early days on that yet.
Article was updated after publishing to clarify malware was not an issue at the time. The issue is the developer and what they did.