I've usually found that the arguments for these properties revolve around, "well, what if I decide to use traditional Mongolian in the future?," which seems like the biggest case of YAGNI I can think of. I suspect their popularity is owed to tutorial authors showing off their CSS prowess.
(I'm also not convinced of the need to flip sizing values for RTL/LTR, but that's at least useful)
Chinese/Korean/Japanese/Vietnamese originally, but computers have made horizontal writing more common on the internet. Modern Vietnamese is also written in a variation of the Latin script, of course.
Mongolian still uses the vertical Mongolian script, though Cyrillic script m has also been introduced in Mongolia during Soviet times and seems to be common inside Mongolia. However, the Mongolian government seems to be moving back to using the original Mongolian script. Furthermore, the Mongolian people inside China never took up the Cyrillic writing system.
Websites can and do use the vertical script: https://president.mn/mng/ http://khumuunbichig.montsame.mn/index.php?home&readnews=572
Google uses the Mongolian Cyrillic alphabet (https://www.google.com/?hl=mn) so even companies that seem to have a page in every single language don't seem to bother with supporting the original script. This is probably because the Mongolian speakers inside China can't make much use of Google anyway.
Funnily enough, Mongolian is one of the few known scripts not only written vertically but also left to right, unlike other Asian vertical scripts (which go right to left).
Forget my question on your users getting utility out of this. Have you ever seen a site, any site, that supports switching to top-to-bottom writing? Against what future are we proofing – a new language arising?
Other examples (e.g. vertical Japanese) aren't too hard to find: https://nishinokensetsukogyo.co.jp https://ok-maru.jp etc.
Almost all extant vertically-written languages are more commonly seen horizontally on the web. Vertical Japanese on the web (from my American understanding) is a design choice, and is notably absent from mainstays like https://www.yahoo.co.jp/
That is to say, it is unlikely that a site's language switcher would opt for top-to-bottom writing.
Edit: the Mongolian president's site does have an English version! ...but it's a completely different site :( https://president.mn/en/. Still, this is the closest to a use case for logical properties I've seen, so kudos.
Using logical properties would make it easier to have a common base of CSS that controls spacing, sizes, etc., that can be used by both language versions, rather than maintaining them completely separately.
> the arguments for these properties revolve around, "well, what if I decide to use traditional Mongolian in the future?," which seems like the biggest case of YAGNI I can think of
...And even this one close example doesn't use them!