Comment by ryandrake
I’ve got a MacBook and Mac Mini stuck on Monterey (12), and an iMac stuck on Big Sur (11). I’m pretty much dead in the water when it comes to software compatibility, unless I want to put Linux on them. Even homebrew gives me a warning that they’ve stopped support and to expect everything to break. It’s a sad state of affairs.
This[1] worked well to upgrade old Macs that were stuck on old versions of macOS for me, if you're not choosing to stay on older versions for other reasons.
[1] https://github.com/dortania/OpenCore-Legacy-Patcher