According to Wikipedia(see Here)

In 2016, with the release of macOS 10.12 Sierra, the name was changed from OS X to macOS to align it with the branding of Apple's other primary operating systems: iOS, watchOS, and tvOS.

Besides, the "X" in "OS X" means "ten", but since macOS Big Sur, the version of macOS is 11, it's not ten anymore, so the name of the OS should be macOS, not OS X anymoe