A classification system by user could eventually be useful. This has been suggested several times for SO and received considerable user support. (The point being to group ‘Favourites’ according to the choice of the individual user – distinct from the general category or tag system, ref: https://meta.stackexchange.com/questions/125180/allow-grouping-of-favorite-questions.) I suspect one fairly good reason why this fell on deaf ears is that it would, for SO, probably require some redesign as well as code. (The best I could come up with was to display not only the star but a single character, chosen by the individual user, within the star.) It must also be possible that it was considered but would have degraded response times excessively.
I don’t consider ‘stars’ as MVP (and see no mention of ‘Favourites’ at https://github.com/codidact/docs/wiki/MVP-Decision-Log) but should the MVP design at least allow for the possibility of adding this feature at a later date, without major redesign?