It was never called "OS/X"[1]. I've never even seen it called that colloquially until today.

1. https://en.wikipedia.org/wiki/MacOS

was it ever called OS/X?

It was called OS X and/or Mac OS X.

But now, not only is it not branded OS X, but it’s literally not at version 10 anymore. (The X was a roman numeral.)

It was a rhetorical question. Yes, I'm well aware it was OS X and that it was never referred to as OS/X