Thanks for the response Aaron. However, that is quite different than the Realmac blogpost that said:
“I’m pleased to say we’ve managed to ensure RapidWeaver 8 will run great on everything from macOS 10.11 through to macOS 10.14!”
That is quite a switch from “will run great” to “runs on 10.11, but this is unsupported”.
Admittedly Rapidweaver 8 appears to run fine on OS 10.11, so far. What concerns me is that I may invest a lot of further time and effort on my website, only to find some problem later on. I guess I will have to decide if I want to take a leap of faith and use version 8, or go back to version 7.