The article mentions Korean but vertical writing is not used at all in Korea. It went out of fashion many decades ago. I'm not sure why safari decided to add this feature...
There's only one language in common use where vertical writing is effectively mandatory: Mongolian. They also use Cyrillic for historical reasons, and Latin script as a modern convenience for electronic communications, but the native script is vertical.
That's enough reason to support it, imho. Japanese and Chinese are also routinely printed vertically, so that adds another billion and a half people who stand to benefit from it.
At least in Mongolia, Cyrillic is dominant and usage of the traditional script is marginal, though there are efforts to bring it back.
Most users of traditional Mongolian script are in China, where websites are in Chinese by default and Mongolian form elements are rare. They do exist though, and it would be nice if the search bar of this bookstore https://mn.dayangds.com/ could be vertical like everything else.
Looks like mn.dayangds.com does UA sniffing so you only get the vertical search bar if the user-agent contains the string "Firefox/" followed by at least one digit. Ugh.
That's enough reason to support it, imho. Japanese and Chinese are also routinely printed vertically, so that adds another billion and a half people who stand to benefit from it.