How to know what Operating Systems have been tested and are supported with a new release of MailStream Plus

Products affected:  MailStream Plus™, all versions
The Operating Systems and  versions tested and supported are listed in the Release Notes that come out with each new release of MailStream Plus™.  
For example:  The 8.3.7 Release Notes includes this on page 23:
Platform-specific information
Release 8.3.7 of MailStream Plus™ is supported on the following platforms:
Platforms          Version
IBM®i              7.2

Linux              RED HAT 6
                   RED HAT 7.X
                   SuSE 11
                   SuSE 12

UNIX               AIX 7.1
                   AIX 7.2
                   SOLARIS 5.10
                   SOLARIS 5.11
                   HP-UX 11.31I

Windows®           2008-R2

z/OS               2.1


Those are the O/S and versions that 8.3.7 has been QA'd on and are supported on.  Although the software may work on other versions due to an Operating Systems' backward and/or forward compatibility, we  cannot  recommend using the product on them, because we cannot guarantee that the product will install, or if it does install guarantee that it will work correctly and/or provide correct results.
UPDATED:  June 27, 2019