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.
You're confidently incorrect. If you weren't paying attention at the time, it's literally in the first sentence of the Wikipedia article. This isn't hard to check.
"OS X" was the official name for five years, from 2011-2016. It wasn't short for anything. The "Mac" prefix had been officially removed from it for that time.
This reminds me of when my grade school teacher kept trying to insist that my friend, named Alex, was actually named Alexander. I think he originally brought in a copy of his birth certificate to shut them up.