It might be fairer to say that a game's mechanics tell us what the writer/s thought the player/s would need rules for.
I get the distinction your making, but I don't know if it's meaningful enough to matter. I mean, the things you need rules for are largely the things that must come up in play, right? Sure, some things that lack rules may come up, but I don't know if this is a case of such things not needing rules so much as being less of a focal point for the game. More that they weren't expected to come up often enough to devote more rules to them.
The minimal social mechanics that did exist in early D&D were still tied to combat and delving.....morale, henchmen, followers, etc. In a pinch, if Robillard needed to impress some NPC, perhaps a Charisma check of some sort could be made. Each edition has had varying degrees of guidance of this kind in the books. But they usually take the form of suggestions, where as the more focal areas read as more codified.
Interestingly, one of the oldest debates in RPGs (since it arose out of D&D) was the debate over whether rules enabled play, or rules discouraged play.
In other words, did additional rules in an area lead to better play by delineation, or did it cause players to be forced to work within the rules and thereby stultify creative play?
I think this likely speaks more about the quality of the rules and how they fit the gameplay more than merely their presence. Yes, certain rules can be an obstacle for any number of reasons. But is it
a rule that's the problem, or is it
that rule?
My group tends to view Encumbrance this way....that it's too fiddly and annoying, and doesn't add to the enjoyment of play, and so we hand wave it. It slows the actual game down and does it for a result that doesn't feel all that meaningful.
Same group of players find the Gear/Load system in Blades in the Dark to be easy to implement, and enjoyable in play. It leads to potentially meaningful choices that they need to consider, but doesn't come with the annoying bookkeeping. In that sense, it enables play.
That's just a simple comparison and anecdotal as all hell, so take it for what it's worth.