AS I said my 2011 MBP Unibody is running High Sierra (10.13.6) without any problem. It would be nice to be able to use Mojave on it but sadly its too old.
On the other hand I have a 2013 Mac Book Pro which came with Mavericks and I advanced its MacOS year by year as upgrades came along to the point where I ran Big Sur from 11.0 to 11.5 when It crashed and would not Recover Big Sur - nor anything down to Mavericks but one Mavericks was on and working I was able to upgrade it to Mojave which I really liked but sadly it crashed on that several times. Presently I have Mavericks installed but not yet complete. The machine came with Apples own SSD but of only 250 GB. That was never truly enough and I suspect the SSD is the cause of the problems especially 'perhaps' due to the extra strain of Big Sur, latest versions were too much! The MBP 2013 was always the oldest Mac considered ok for BS. I hear rumours that Cupertino is seeing/hearing of many more folk with problems like mine. I am planning to get the local Genius Bar to test it for me so that I can find a solution to get it working again.
ColinY