A customer wanted to know what happened to the Arial Unicode MS font. It used to be installed with Microsoft Office, but was dropped starting with Microsoft Office 2016. Various conspiracy theories have developed as to why.
Wikipedia cites a Microsoft forum answer that says that
When Microsoft included Arial Unicode MS with earlier versions of Office, Microsoft paid a licensing fee to The Monotype Corporation, which is the copyright holder for the font. Someone at Microsoft decided it was no longer worthwhile to continue paying that fee, so it was removed from the Office package. The price you see now is what Fonts.com charges for a single copy.
The fonts team provided the correct answer: Arial Unicode MS was included with Office at a time when applications did not handle font fallback properly. It was essentially a last-resort font that someone could specify and get relatively reasonable results. As Unicode grew in size, it became clear that the font could not hold every defined character, and it also became difficult to maintain because its size pushed various tables to their maximum. Adding new characters became impractical. Consequently, it was deprecated, since it could no longer perform the job it was originally created for.
As for the font ownership: The answer above has it backward. The font is wholly owned by Microsoft, who licenses it to Monotype.