A newer RSAT version is always backwards compatible with older server OS versions. For example, all Windows versions from 2012 R2 and below are fully supported by RSAT for Windows 10.

Older RSAT versions can manage newer server OS versions, but only features that are also present in older OS versions can be managed properly. For example, RSAT for Windows 7 partially supports 2012 R2: features that are present on 2008 R2 can be managed, but features unique to 2012 or 2012 R2 cannot).