[TwiceCubed]

They didn't "just stop writing it" in some contexts. They started registering trademarks that explicitly didn't include it, and then they went back.

The trademark for "OS X" on its own was approved in November 2011, and then Apple started using it on its own. Their other trademark registrations (e.g. USPTO TM SN 86097345 OS X Mavericks) continued in that form, until their registration for "macOS" was approved and they changed back (e.g. USPTO TM SN 87534929 macOS High Sierra).

You can have your etymology and your opinion, but as far as Apple and the government are concerned, the fact is that the official name was just "OS X". Wikipedia is correct, and this was also the understanding among Apple fans at the time.