The history of names for 10.X, from the reference above (which also has nicknames, like 'Mavericks' and 'Mohave', which I believe we do not use).

10.0 to 10.7: Mac OS X (space or spaces sometime left out, as in macosx)
10.8 to 10.13: OS X (2012, ditto)
10.14 to (current) 10.16 (2016)

AFAIK, the current Mac installers support either 10.6+ or 10.9+.  The macosx references to 'Mac OS X 10.6' and 'OS X 10.7 and 10.8' are properly not changed.

'OS X' in pyshell.py is removed in PR 10464.

The most recent item in NEWS.txt using 'OS X' is for #27310, June 2016, by Ned, who used 'OS X'.  So no changes to NEWS.txt or NEWS2.txt.

** Ned **
Should 'OS X Aqua Tk' be changed to 'macOS Aqua Tk' or 'macOS (OS X) Aqua Tk'?  In other words, is 'OS X' here a replaceable noun adjective or an integral part of a proper name?  The PR did the 2nd replacement in config.py, since the comment is about the 'IDLE Classic OSX' keyset, but I deferred the change in mainmenu.py.

Same issue with 'OS X Cocoa Tk 8.5' in editor.py.

Editor.py also has "Some OS X systems have only one mouse button".  It seems to me that 'Mac' would be a better replacement that 'macOS' as the mouse is part of the hardware system, not the OS software.