Had Apple given their OS a proper name, I assure you people would use it. You shouldn't use the "OS" moniker as part an OS's name. It's redundant, like naming your child "Human Child Jimmy". Obviously people will take all sorts of shortcuts around it, which you will not have control over, leading to weird "HC/Jim" and the likes.

It's a bit like "U.S.A." not being a proper country's _name_. I mean, would you name you children "Coherent-Enough Assemblage of Bodyparts"? You keep that long-form stuff for the _description_ field! Somebody fucked up when they filled out the form, ain't no fixing it now.

Why do people insist on calling “X” “Twitter”?

When something has a name for 15 years, it tends to be pretty sticky.

“OS/X”, though, that’s just someone messed up and jumbled two names together.

Not everyone cares about the marketing departments feelings

Rebellion against meaningless name changes. Why does Apple insist on calling Mac OS an OS X, Mac OS X, macOS?

I mean, muscle memory & fatigue may be part of it. I used MacOS for a number of years, then I used Mac OS X for 8 years, then OS X for 5 years, and now macOS for 9 years.

During that time I also used Windows and Linux from time to time. Their names didn't change in a way where just calling them that was perceived as incorrect.