Outlook was first introduced in Office 97 which had the version number 8.0. The version numbering of Office itself skips number 5 and 6. This is because since Office 95 (version 7.0), all Office applications started sharing the same version number. Word at the time was in its 6.0 version and with that it had the highest version number. Its next version for Office 95 would become version Word 7.0 so that version number was taken for the entire Office 95 suite.
I have a MacBook Pro (15-inch, Late 2011) that uses High Sierra 10.13.6. I believe that this is the highest OS available to the machine. No more upgrades available. When I try to install Office 365 on my MacBook Pro I get a message "This software requires macOS version 10.14.0 or later." 2b1af7f3a8