Having raised my query within the community support section, I have been advised by @v-yutliu-msft to raise it here also as a feature suggestion.
I am sure I am not the only one who is trying to put together collapsible tree menus. I am using these to display a lot of cost data and I don't think that there is any way to accomplish this without using multiple nested flexible height galleries. Currently, Powerapps only permits a single nested gallery (I understand that multiple nestings were removed a while ago).
I appreciate that there may have been good reasons for removing this feature, but if there is no alternative method to accomplish collapsible tree menus, then I strongly suggest its reintroduction, perhaps in a different form geared to this specific purpose.