I mentioned above how Apple had to pull
the 2020-005 security update for Mojave because of major bugs that came along with it. CPU utilization would spike, the fan would sound like a jet engine, and programs would be sluggish or randomly crash. Ostensibly, this was a minor security update that should have had proper testing beforehand, but Apple pulled it almost immediately, because it was clearly not ready for a full release.
A similar problem happened where Safari would kernel panic using a 2018 Mac mini running Mojave (not Catalina) and the only fix was to update to the Big Sur firmware (which was in beta, at the time). It wasn't officially patched until months later, when that firmware version shipped with Big Sur and trickled down to Mojave in a security update. This happened to everyone with a 2018 Mac mini
who was running Mojave after applying security update 2020-002. Because I needed to continue to use Mojave, at that time, I switched to Firefox, until I ended up installing the Big Sur beta firmware so that I could use Safari again.
Last year, Apple finally backed up what myself and others have been saying: only the most current version of macOS receives 100% of the latest security patches. What Apple doesn't tell you, and never will, is that the oldest versions also receive the least amount of testing, assuming there's much testing done at all. Those two massive problems with Mojave didn't happen in the most recent shipping version of macOS, at that time.
Now, it's Big Sur's turn to receive inadequate testing. While not nearly as bad as what happened with those Mojave updates, Big Sur 11.7.3, the latest security patch, prevents the icons in Safari's Favorites section from appearing properly
View attachment 21766
This is yet another reason that I think it is best to run the latest version of macOS, whenever possible. The old school way of thinking is that once a version of macOS hits maintenance mode, it's essentially "done", and only requires an occasional minor security patch. Those "minor" patches often touch major parts of the operating system, and I don't think Apple tests them nearly as well as the most current release.
The vague, roughly two years of additional security patches often seem like an afterthought. Apple won't officially commit to fully patching these old versions 100% and don't appear to test them as rigorously. I suppose it's better than nothing for Mac users who are still stuck using an older macOS release, for whatever reason, but the tradeoffs should be clear.
So, it's not just the lack of 100% security coverage that is an issue, but it also appears that Apple doesn't test those older versions as well as the most current release of macOS, either. I never imagined that I would have to install beta firmware to properly use Apple's own web browser, but that's exactly what 2018 Mac mini owners needed to do a few years ago.