USA is a big place, but OP is talking about NYC specifically before COVID. At that time you were liable for the full amount until the end of the lease. Landlords could sit on empty boxes and sue you for the entire amount. Whether they want the hassle is up to them, but you couldn't compel them to do anything.

It does sound unfair, but see here for supporting sources https://www.nolo.com/legal-encyclopedia/tenants-right-break-...

Your own link says that NYC landlords are required to mitigate damages by seeking a new tenant.

That is a recent change in NY law, until last year landlords werent required to mitigate and rerent, it was on the tenant to sublet

They could put an ad on Craigslist and deny all applicants or get non either way sought a new tenant.

And doing so would be illegal, and you could sue them for it (and win). It's pretty easy to determine if a landlord is actually trying to rent out a given apartment -- just have your friends apply.

Your “pretty easy” sounds extremely onerous to me. If my friend said, “hey, apply for my old apartment so I can prove in court that my landlord is fielding/dodging applications.” I think I’d tell them, unkindly, to go F themself. (Yes, That’s how I talk to my friends, but also because I think it’s preposterous that that even asked.)

Going into court you have no/low knowledge of what your landlord has actually done. If they pull out their ads they posted on the internet or on some zero readership newspaper. The judge will see they have made an effort and you probably are grasping at straws because you have nothing in the way of proof. If you’re lucky, judge would hear some expert testimony (realtor) that says the apartment/market/etc should have been filled within 1-2 months and they feel it’s highly unusual the unit would remain unfilled for 6 months. But even still, if the law says the landlord is required to market the property it might not imply they have to be good at marketing the property. So there’s a grey area where it could go either way, and he can proof that you can’t match.

Yes, as of July 2019, which is not the context of the OP discussion and why I said "at the time..."

July 2019 was pre-covid?

Yes.