Developers who want to keep a particular FileMaker version and need to upgrade their machine's macOS to a version not supported by the FM version in question, can install that FMP in a virtual machine like UTM.
The UTM website provides a detailed step-by-step description of how to set up a virtual macOS host on which the FMP can be installed.
I also use virtual machines on UTM for FileMaker Server. I always set up a new VM when a new FMS version is released. If there are problems with the new FMS, I can always switch back to the previous VM.
Thanks @Torsten! I am still using Mojave on one of my workstations for the reason of old versions, but will now try this out 
I also like to remind everyone, that you can keep older FileMaker versions on current macOS and still run them from time to time to check something. Yes, they may show a few issues, but mostly still work. My current collection here:
On Windows I do the same by duplicating the FileMaker app folder and then rename it. After that I can uninstall old version and install new version. This way the installer can connect .fmp12 extension to latest version.
can you run multiple versions of FMP on Windows at the same time like on the Mac?
Yes, of course.
great thanks - how about multiple copies of FMP20.2 to be able to test multiple privilege sets accessing same hosted DB?
But on Apple silicon can you run an older version of macOS in a VM, to support an older OS instance for as earlier FMP version? I believe to support older FMP versions, MacOS may not be a valid approach???
Most older versions of FMP run in the latest MacOS - Claris only indicates that they have tested compatibility with supported versions, not that it won't run in a newer OS - YMMV as some features may indeed NOT work as expected.
An automator script is an option - open one copy for test and another for development, concurrently:
On an Apple Silicon Mac, you can run a Windows VM with various older FileMaker versions, down to version 6 if needed (or even older?).
Directly on the Apple Silicon Mac, you can run the Mac versions from 14 to the latest one. I have them all installed here and it works. Not necessarily perfect as older FileMaker versions show various bugs on newer macOS.
That may help. If the clients are complaining about "I see condition X", it may be the old version on the new OS problem.
