Recently reinstalled a virtual machine with FileMaker versions to test:
Good news is that those all work on Windows 10 and allows me to check if my plugin works.
Especially if you have a problem with an older version, I can try myself here. Although I know that over 90% of my plugin users do use FileMaker 13 or 14, so bug reports for versions before 11 are rare.
But by using special code which is not in the official FileMaker SDK, I can keep my plugin load in older versions and still use newer SDK features if available.
PS: Still waiting for first bug report for FileMaker 15...