> What do you mean with the "uppercase legacy"? You mean all the current documents we already have?

Specifically what motivated this post: collections of snippets that
have been written with the historical convention in mind. It's easy to
convert them but it's not that easy to convert users :).

I am one of those who prefer the lower-case keywords :)

I started converting my Org documents to use the new convention around the time it landed in the Org mode master branch little more than a year back: https://scripter.co/org-keywords-lower-case/.