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As the article notes, not all models have that option. 2017-or-later is the cutoff now I believe.

Sure, take the upgrade if you can though.

Who can stop me from compiling and installing some software though?

Installing the newer versions won't make the old ones go away though, and there are presumably a lot of built-in things linking to the old versions.

Sure, you can side-load it. Stuff might break since the OS makes assumptions about the hardware.