Xfce has libxfceui4, which should be equivalent to libadwaita (I never directly used it, but that was at least my understanding of it)
Then I guess you missed the huge pile of mess that CSD made when they were first introduced to Xfce.
I guess I did. I didn’t know what CSD was until just now. Is this something that using Xubuntu has shielded me from? I’ve been using it since around 2016, maybe a bit earlier, (using the default theme) and it’s never seemed to be a problem.
I don't use xubuntu, but I don't think so. I don't see how xubuntu wouldn't be affected since this is a GTK+ issue. Starting any CSD application in Xfce will behave and look out of place and unfortunately, even some Xfce applications and tools used CSD (perhaps some still do).