The add Object -> Collection -> New Collection (Ctrl-G) seems to be leftover of the changes when introducing collection (which ar not groups). In fact in Object Properties -> Collection there will be a new collection.. and if switching the outliner display mode to Orphan Data you also can see them.. but they are not…
When you do Ctrl+G it makes hidden collection not showing in Outliner View layer . Only in Orphan data . I personally find it quite convenient for some service things , temporary grouping, testing etc. if you use such group in geometry node it's not orphan anymore but still hidden in "view layer" and not in "Blender file"…