Edit: A lot of people say, that GWM needs a melee weapon attack, but they miss Jesses point: While GWM requires a melee attack with a heavy weapon, Sharpshooters only criteria is an attack with a ranged weapon (not a ranged weapon attack). Jesse bases his claim on the fact, that a crossbow is still a ranged weapon, even if used as an improvised weapon for melee combat. That’s why it deals 1d4(!)+20 damage. (It works with any ranged, heavy weapon btw., so Longbow qualifies too.) Of course Jesse is playing the devils advocate here and of course, no somewhat sane Walter will allow this in any campaign ever, as it’s obviously not the intention behind these feats. But you could read it that way and that’s Jesses (paperthin) point. Besides: he finds the image of a barbarian running around recklessly smashing a crossbow over everyone’s head to just be hilarious.
I think if one is being nitpicky, the reading of the Ammunition Property is that it specifically calls out weapons with ammunition as being treated as Improvised Weapons (which would come with not being able to use the weapon's properties or proficiency, or at least so it seems to me), rather than redirecting you to read a different part of the rules section. It would be odd (and honestly it is a bit odd with the normal reading as well) to have two distinct properties that qualifies something to fall under the same rule (being a Ranged Weapon and having the Ammunition Property), particularly when one is always going to contain the other.
And the Thrown property on the Dart being useless only really becomes a problem if you take this very specific and nitpicky ruling to be good, which honestly mostly serves as evidence that reading the rules that way is not RAI. It seems to be the RAI intention that the Dart having the Thrown Property is to allow you to use your proficiency for the attack, which you would not be able to if you threw an Improvised Weapon (such as if you were to throw a Sickle).
That's on me, I've been playing my tavern brawler for too long and overlooked that most people don't have imrpovised weapon proficiency. It looks like using most ranged weapons in melee is maybe improvised for two reasons? Like, the ammunition property makes it improvised, but also the "ranged weapon to make a melee attack" rule makes it improvised. Which I guess lines up if you take it as Javelins being good for melee and throwing, while darts are only really good for throwing- makes sense to me, although it's weird to have the same thing said twice over (a ranged weapon is improvised, but also an ammunition traited weapon is improvised, and only ranged weapons have that trait so they're already improvised in melee)