Where on earth are you getting this idea?
Halflings probably use slings a lot, for reasons that others have already gone through, and mostly because they got a bonus with slings (and other thrown weapons) all the way up until 3x. This does not mean that they won't also use bows or crossbows. Or swords, or axes, or anything else.
Halflings do not have to use slings at all, if the DM wants them all to have bows or crossbows instead. In fact, halflings can have halfling-sized blunderbusses if the DM wants them to.
From Oofta, who objected to the idea that halflings use crossbowx, because they are a weapon of war. Followed by you putting forth this idea that halflings have trained their whole life in the use of the sling.
Which seems to be an artifact of older editions, and not 5e. As I have stated, the word "sling" doesn't appear in any halfling entry in the PHB or Mordenkainen's. This supposed affinity for slings no longer exists. And the other reasons people have given is because Slings are cheap. But saving money isn't exactly something we expect from Halflings, they aren't hoarding wealth, so who cares if it is cheap.
Light and not in the way? Okay, that shouldn't be your first concern when planning out a village defense. Personal Defense? Sure, and I stated at the beginning I liked the idea for personal defense. Just that it wasn't adequate for defense of a village.
Wow, it's like we're playing a game that entirely consists of things that are made up! Forget halfling-sized blunderbusses; halflings can have laser rifles if the DM wants them to.
And you seemed to have missed the point entirely. Let me give you an example
"Halflings have slings and can fight off the gnolls"
"Well, gnolls have bows in their entry, so they outrange the halflings, and they'd have to get really close to use the slings"
"The halflings have tons of 4 ft high cover, they'll hide behind it and snipe the gnolls, and the gnolls can't shoot them behind the cover"
"Where did this cover come from? Anyways, if the halflings are pinned down behind cover, the gnolls can either just circle around it or keep them pinned and have some get into melee. The Halflings would be impeded by the cover too, making it harder to take out the gnolls without getting shot"
"Well, just give the halflings the sharpshooter feet in regards to slings, then they don't need to worry about the range or the cover."
See the issue? I say slings aren't good enough, now halflings are getting a feat to make the slings good enough, instead of just... having them use a different weapon. Yes, the DM can make up anything they want. But once you start giving out feats to an entire race of people, you've started altering the game balance.
No, it doesn't. Because slings are a simple weapon everyone gets--unlike the martial weapons that elves and dwarfs gain proficiency in--and the 5e writers didn't choose to give them any other bonus with slings, like ignoring cover or increased range or damage. It doesn't actually break anything to say that halfling NPCs typically train with shortbows or crossbows or whatever.
Right, and crossbows are more effective weapons for them to use, and easier to train in the use of. So they are a good choice for defending the village.
That was my entire point. That right there.
And, if "everyone" gets proficiency in slings (which they don't, only every PC gets that) then humans and others could use slings every day too, because they are cheap, and easy to carry, and effective defenisve measures. But, I'm being told that no they can't, because humans aren't community minded, they'd do a militia and have specialized soldiers instead of having a way to defend themselves. Which... makes no sense. It is just to defend this idea that this sling idea which has no support in the 5e lore is somehow uniquely halfling.