Firefox has an LTS version which can be a baseline for features, but which receives security updates at the very least.
I don't know about, say, Chrome 80 line that would receive security fixes but not new features. I also think that would be against Google's interests to have such an LTS line, it would decrease the moat between it an other browsers.
Because supporting older Chrome releases is pointless: they should be replaced because of their security holes, discovered later. You may, of course, to decide to put the baseline somewhere, but that baseline would be arbitrary.
With Firefox (ESRs are at 102 and even 52) and, to an extent, Safari (tied to iOS releases) there are non-arbitrary baselines, where you know what kind and size of the audience you additionally cover by staying away from newer features.
They point is the same for Chrome - "should" is a wish, not a description of reality, so if you care about users using old versions, you use a fallback method.
Also don't you have stats by version number, what is extra non-arbitrary about LTS?
Firefox has an LTS version which can be a baseline for features, but which receives security updates at the very least.
I don't know about, say, Chrome 80 line that would receive security fixes but not new features. I also think that would be against Google's interests to have such an LTS line, it would decrease the moat between it an other browsers.