@xyzzy You are right about those two. What it looks like is that Chromium supports the older unsupported OS version for two years after OS vendor support ended.
In some ways Win 7 was an exception, but that was because Win7 business users could pay for support until 3 years after public support ended, and the Chromium support was ended as the paid support ended, alongside Win 8.1. (There is a similar 3 year paid support system announced for Win10 when public support ends in a year.)
So, .... In a year we will probably be having this discussion again, for MacOS 11