The X was dropped in 2016, with macOS Sierra. Maybe because they needed the X next year for the iPhone.
Man, after turning 35 and having kids, all years just blend into one ;)
That was dropping the X from the name, not changing the major version number from 10 to 11. It's had a major version number of 10 ever since the first release of OS X. Most people assumed the actual version number would stay at 10.x.x forever, regardless of any changes to the marketing name of the OS itself.