OS X (the name) is gone for a while, it was replaced by macOS. The numbering change now is inconsequential.
I.e., IBM becomes Ibm, IKEA would be Ikea, and macOS would become Macos.
The reasoning is that companies shouldn’t be able to dictate flow and attention in an article by manipulating capitalization in brand names.
(Just an anecdote, obviously not applicable to comments here)