Dear somewhat anonymous "IT Admin UC4",
the compatibility matrix is indeed the definitive, binding ressource.
Older agents will probably work on Windows Server 2016, too, but you can not expect Automic to provide any support unless it's, well, officially supported.
Either way:
The current version is 12.1, and while Automic is not very clear on what they consider a "major" or minor release, as per most sources they did consider 11.2 a major release, and support only the last two major releases. By the official documents, maintainance for 11.1 has already ended in February 2018, and support (no, I don't know what the difference is) will end Feb 13 2019.
So you may want to use the highest version your engine supports (and ideally the one matching your engine), not the lowest one possible.
Hth,