I don't deny they could - I consider it unlikely that they would. Or at least those who would are extreme outliers. The Ballearic Slingers that the Roman army used as auxillia might have done this - but only at the point that the Romans were using them as mercenaries so everyone learned it because it was the best way of making money (which is far from the reason halflings do).
So I'm saying it's vanishingly unlikely that humans would do something that humans would do as a society something that as far as I am aware that no known human society has ever done. But it's something that completely fits with halfling mindset and values.
Of course I might be wrong and that there have been human societies where everyone carried a sling - and I will gladly drop this objection if you are able to provide the historical research showing me to be wrong?
Okay, so you want to immediately pivot from the game world to the real world. Okay.
How about these three right here. One is a forum of experts on war, one is a reddit used by historians, and one is a cited, researched article
Official hub for Creative Assembly games. Interact with fellow gamers, learn about the latest features and earn your CA badges!
forums.totalwar.com
Do you know what all three of them say? In the very first response in the forum and the reddit.
"Slings are highly skill intensive, more so than bows or crossbows."
So, no, in the real world, no human community had everyone carrying slings, because slings are the most difficult of the three weapons (slings, bows, crossbows) to learn how to use effectively.
From the article "Reports of estimated range of the sling varies in recent literature. This may stem from the inability of historians to find individuals who can properly demonstrate the sling. The bow, crossbow and firearm, if operated correctly, will produce the same effect the weapon had hundreds of years ago.
However, the sling requires tremendous skill, and only people who have had extensive training can claim to match the ability of ancient slingers. Existing literature quotes ranges as little as 150m to as much as 500m (Demmin, 1964; Hogg, 1968; Korfmann, 1973; Wise, 1976; Connolly, 1981; Ferrill, 1985; Richardson, 1998b)."
OH! and look at this. What serendipity, from the same research paper
"Strabo, a Roman historian born in 64 B.C. commented on the famed Balearic slingers:
...their training in the use of slings used to be such, from childhood up, that [parents] would not so much as give bread to their children unless they first hit it with the sling.
Vegetius, Florus, and other classical writers confirm this Balearic tradition and their remarkable proficiency. The Bible also mentions another legendary group, the Benjamites, noting, “every one could sling stones at an hair breadth, and not miss.” (Judges 20.16)"
So, guess that according to our best sources, we have two human societies where are large number of people used slings. The Balearics and the Benjamites. So, historical research proving you wrong. That was a fun bit of learning.
I no more consider that than that I consider that dwarves mostly being miners means that there are no human miners in a D&D setting. Races in D&D take archetypal things about different subgroups of humans (because humans are the only sapient species we have to base things on and everyone is playing a human) and turn them up to 11.
And yet the idea that a human community would band together in mutual defense instead of relying solely on the militia, is met with scorn, because they have a hierarchical society.
If they are all in their own homes who would they need to defend that's outside their homes?
The people in the homes being broken into. Guess that community spirit doesn't apply to saving your neighbors.
Based around long term halfling lore and that changes no game rules at all.
Lore that was written out of the game. And I guess you've abandoned you "give all halflings sharpshooter with the sling" position, unless you think that isn't changing game rules.