In my experience with 5th edition, usually any class expected to have a decent Charisma fills the social interaction niche. Sorcerer, Bard, Warlock being the top three with Paladin, Rogue, and Clerics sometimes stepping in.
I haven't done it with class, but seeing how it worked well in Baldur's Gate 3 I might start doing it. But I like giving players advantage because it makes them happy to know what choices they made creating their character matters and Advantage is an elegant option for a lazy DM like myself.
Have you ever run into a situation where a player already
had Advantage, and thus you didn't have anything else you could give them?
I was more thinking that some players are okay with, even more comfortable in, a sidekick style role. Like playing Lois or Jimmy to Superman. They are still on screen a lot, but not necessarily the mover.
And they can elect to have that in a system that isn't balanced around a spotlight. Games that are predicated on a spotlight, however, do not allow for addressing the problem in ways that don't use the spotlight.
I don't know 4E well but didn't it have like 4 PHBs and DMGs? What was in those?
Additional classes, races, and feats for the former (3 PHBs, and the two "main" Essentials books,
Heroes of the Forgotten Kingdoms and
Heroes of the Fallen Lands, which print what 5e would call alternate "subclasses" and "subraces" without printing the originals). The latter just adds more DM-side advice, useful DMing tools that aren't really rules, and reference materials.
To the best of my knowledge, no books added anything that I would consider a new subsystem. The closest thing might be like...the inclusion of more specific survival mechanics in the Dark Sun Campaign Setting? But I'm not really sure that that qualifies, since that's really more just...additional heft to the existing 4e mechanics for exhaustion and reasons for triggering it more often, IIRC.
Also, technically speaking, 4e had a policy that everything (first-party) was core. All the rulebooks, all the magazines, all the adventures, all the game day fun stuff. If it was in the Compendium, it was core--period. Whether you consider that mere sophistry or a serious commitment is up to you. Certainly, they did a very, very good job of keeping things balanced
as if every book, magazine, etc. were core.
Not choosing to use it is different than the game not having it.
No, I mean like...if you stick with the original publication, there never
were any. For example, the "Black Box" by Troy Denning. Rules Cyclopedia was also published that same year, but it's a separate product, lacking some of the low-level features the "Black Box" offers.
The "Black Box" really does just sort of...stop...at level 5. You can convert over to the Rules Cyclopedia (it isn't hard, they're
nearly the same up to level 5), but there really are differences, or so I've been led to believe.