Freezing background apps just isn't needed all that much if you run a fully FLOSS system. It's much more of a concern for proprietary software where you don't have the source code available. There's a similar story for sandboxing actually, it's not a coincidence that it's been getting more popular as proprietary apps have been made widely available via Flathub and the like.
There is a reason why Pinephone and similar run hot as hell for a couple of hours of uptime only. But Linux laptops also have a terrible track record here. It has nothing to do with privacy, it's purely there to properly save energy.
And come on, Linux Desktop has terrible security, just because no one targets the 3% marketshare doesn't mean that they are safe at all. Especially that security is independent of "proprietariness". You can have, say, an open source PDF reader with a vulnerability - you only need to open a malicious PDF file to have your system corrupted. Putting our heads into the sand is not a good idea.