There are 2 ways of possible grouping.
- Static context - collection can be turned to a Group in case if it is non-cumulative.
- Dynamic context - a separate cross-hierarchical entity that groups selected objects (Common Group paradigm, like in your proposal or almost every software, except Maya).
Indeed, grouping was discussed for a long time in different treads.
They was announced as well (it was time when Ctrl+G shortcut was reserved for grouping ability)
My assumption is that such an ability was ignored during 2.8 depsgraph redesign (probably because Maya don’t have Common Groups paradigm), and it turned to be too hard to implement Groups later into resulting depsgraph realization.