However, it was unsupported. The IT lesson is always that "unsupported" is not the same as "doesn't work". Supported means, in this context, that Apple will update the OS with a variety of ongoing security updates and maybe even a feature update. This latter possibility being unlikely.
The supported OS was High Sierra. This is the 2017 MacOs. To be fair none of this is important to me specifically. Getting a really cheap 11 year Mac working was the exercise, not what OS was installed. The fact is that I got quite a few browser crashes and, from what I could see, a considerable performance hit from trying to get it all working.
I am off work and locked down. So today I re-instated High Sierra, version 10.13 of the Mac family.
In the paraphrased words of Clint Eastwood; "A man has to know his limitations". Today, my limitation was High Sierra.