The #categorytree magic word doesn't seem to work in VisualEditor and Parsoid.
If I have an article in the English Wikipedia that has the wikitext {{#categorytree:ASCII}}, VisualEditor shows it like this:
Similar output appears if I use the 2017 wikitext editor and show a preview.
In a rendered page, the same wikitext is shown like this:
It's probably not important to invest a lot in detailed support for #categorytree in VE. For example, it's not very necessary to have an item that inserts it in VE's "Insert" menu. However, it should at least be correctly rendered while editing and previewing, and not shown as escaped HTML. It would also be nice to be able to change the parameters of an existing #categorytree instance, like mode and depth.
It is probably not so commonly used in English Wikipedia articles, but it is quite common in Russian Wikipedia articles and perhaps in other some other wikis.