The viewport value for the meta element was never a part of HTML 5.

Apple first invented it for the iPhone 1, published it in 2007, and other manufacturers of mobile browser then copied it.

https://developer.apple.com/library/archive/documentation/Ap...

At the time for iPhone 4 the viewport meta thingy was an old thing, but of course a lot of websites hadn’t updated in the previous four years.

The CSS Working Group has for some years specs in preparation which would make the viewport thingy official:

https://drafts.csswg.org/css-viewport/#viewport-meta