Privacy

It does not matter what games you are running or whether they have anti-cheat. If you are running a program without any sandboxing, they can access everything on your system that your user has access to.

Steam launches games (doesn't matter if you use Proton or they are running natively) as a subprocess and if Steam itself is not sandboxed, those games will also not be. While the symlinks may be abused for this, Wine also gives access to running apps by mounting your Linux root folder as the Z drive (similar to C or D drives you may have already seen).

Running any untrusted software on your system (particularly proprietary ones like most games), you should be mindful of associated privacy risks.

I also wanna address one thing about the particular games you mentioned: EAC and BattlEye run with elevated privileges on Windows. This means it can access and modify things on your system that your normal user can't, like changing core system parameters (and even BIOS). On Linux, though, they run as your own user with regular privileges. This gives them access to your system only as much as any other app you are running. This is the key reason why some game developers don't let you play their online games on Linux, even when the anti-cheat system they use explicitly support it.

Supported kernel level anti-cheat (EAC and Battleye) are not actually kernel level under proton on Linux. They run only in userspace, which makes them less "effective" than on Windows. However, you said yourself that wine are proton are not designed to be secure, and while I'm far from a professional who can point to anything specific, I'm sure that there's still plenty of non-kernel-level info that they're able to suck up and send back.

Proton is not a security feature. Virtualization is not a security feature either

Why would you make this a proton/wine convo when you are clearly unhappy with the practices used by the companies making the software. This isn't about the compatibility layer you're using, reframe your thinking. If you ran malware, Proton would say "yeah sure bud let me get that for you" and would run that shit. Proton isn't an antivirus or a security tool.

Also, privacy and security are completely different. Privacy is the right to not be observed, think anonymity. Security is the control of access, think authentication and authorisation. If you're attempting to run software on your user account, you've authorised it to run as you. Therefore every action you do that doesn't require a password is possible, if improbable. But yeah, they can see other open software, it can view your files and it can see your hardware. It needs to. It's a compatibility layer. It's still just software you ran on your machine.

I repeat, if you run spyware, you get spyware. You've just given it the tools to work cross platform.

Symbolic links are like Windows shortcuts. You shortcut an address and it takes you there.

The problem comes in abusing both, where a shortcut takes you, and how shortcuts are assigned.

When programming for security, you never use shortcuts, as they are extremely easily to manipulate. Thus when Proton symlinks, it insecurely references files and utilities, which can allow espionage to be performed without your ability to trace it.

It's not that shortcuts are espionage, but they make espionage easier to perform.

Any executable you run is capable of spying on your system unless sandboxed, containered, vmed... Etc.

As a fun aside, if you write some custom assembly into a windows program to issue syscall instructions directly you can issue Linux syscalls from inside windows binaries running in wine.

I answered below already, but Windows infostealers and spyware can absolutely work in proton.

It’s no security beyond some file paths being different, from the software’s point of view.

What I tend to do is run dedicated partitions for games and other entertainment, where I could wipe them without consequence, and if they got infected with infostealers or spyware there wouldn’t be anything to get anyway.

And then the partition where anything sensitive is done/saved (like finances) is separate and inaccessible to that boot.

Is it feasible to run eg.: Steam, within a (s)chroot that does not have host /proc and /sys mounted? That wouldn't do that much by itself but I guess it would help more when combined with running as a different user (and not mounting host /home).

Gotta love how almost every post now has somebody accusing somebody else of using or being an LLM. More so when they're wrong most of the time.

I've said this before, I'll say it again. The AI accusers are worse / more annoying than the actual AI problem.