aka NET_WM_WINDOW_TYPE_POTATO Currently Slack is not showing on the alt-tab cycle menu. This is due to Slack setting NET_WM_WINDOW_TYPE = NET_WM_WINDOW_TYPE_MAIN, which is a non-standard value. Xfwm should gracefully fallback to doing something sensible when this happens.
Olivier Fourdan referenced this bugreport in commit 22fe30cdd99d111f043b3282a02f39df468c7ac2 netwm: Fix window type for broken clients https://git.xfce.org/xfce/xfwm4/commit?id=22fe30cdd99d111f043b3282a02f39df468c7ac2
Seems to fix Slack for me.