[1] https://www.phoronix.com/scan.php?page=news_item&px=Spectre-....
My gaming PC, a Sandy Bridge i7-2600k, has had 20 microcode updates between 2010 to 2019. The majority were in 2013 until the spectre mitigations came in 2018. The last bios update for my mobo contains microcode 2 versions behind the spectre updates.
Feed a BIOS image in to this tool to view microcode information. https://github.com/platomav/MCExtractor
mitigations=off works at the OS kernel level to patch out some expensive aspects of what the kernel can do (possibly in concert with the CPU but driven by the kernel). This issue a microcode change that affects the CPU behavior without any intervention by the kernel, so mitigations=off doesn't help.
It is purely a microcode fix for an "old-school" data dependent timing issue.
I know this way of disabling mitigations on Linux (nevertheless thank you, I well might be unaware of this feature, I only learnt it some years ago).
However, I doubt you can disable microcode-level mitigations this way. I believe it only affects the kernel level.
[0] https://wiki.archlinux.org/title/Microcode#Early_loading
No. If it wasn't impossible, it would add a massive delay to context switches between processes as the required microcode changes are applied.
This is not as sure a deduction as it once was.
The main reason for games not to be playable on Linux is the anticheat software used in modern online games. Pretty much everything else will now run perfectly (see protondb.com). Since the commentor you were replying to specified that they played old single player games they would probably be fine with just Linux. Indeed many sufficiently old Windows games are now easier to play on Linux than they are on modern Windows.