[hiddendoom45]

There's also the risk of accidentally hitting the upgrade button and at best losing a few hours to the upgrade or at worst several days as you deal with recovery.

Had a recent bad experience when I accidentally pressed upgrade on an old macbook air running Catalina. I ultimately had to restore from a time machine backup to get it working again. The internet recovery failed most of the time and needed multiple attempts to boot through. Attempts to try and let the upgrade go through would always result in the estimated time remaining disappearing after a while, and nothing happening. Attempting to re-install Catalina from the recovery menu also failed with an infinite loading spinner after a while and nothing clickable. Additionally there is no indicator that the wifi disconnects and you need to re-connect when attempting to re-install. I only found that out when I accidentally triggered an accessibility setting(I think VoiceOver) that showed the top bar again with the wifi icon. Even then there was another workaround needed to actually connect since I believe it wasn't possible to type in the password as you couldn't select the password dialog box.

I wasn't completely without fault, I'd stopped the upgrade early to since the upgrade was taking too long, I also lost the recovery partition the first time I tried and failed to reinstall which meant that internet recovery was the only way to boot the mac without it immediately trying to upgrade or re-install. The cli is accessible from the recovery mode which did allow me to take an additional backup of the laptop's contents with more up to date things.

In general it seems that the upgrade and recovery process is not supported at all after a couple of years which makes these upgrade notifications a constant risk if you ever click to install one of them. It also reminded me the importance of keeping backups, especially of the filevault key.