[salamandersauce]

The last High Sierra update was 7 months ago and it's now unsupported. It's not going to get anymore now that Big Sur has launched as Apple only supports 2 versions back.

So no security updates for starters, software slowly starts to leave behind old versions too.

[alphabettsy]

I’m not aware of a recent EOL Mac OS that did not continue to receive security updates for some time.

[salamandersauce]

And that time is two years after the next version comes out. That's how it's worked since Mountain Lion and High Sierra's time is up which is why it hasn't gotten any for 7 months while Mojave has received some.