no longer available

Sometimes, perhaps, but it could also be that the seller changed their mind for some reason or other; somehow I even heard about someone who took their unit off the market at least temporarily because of an out of town guest.

No longer available could mean the owner pulled the apartment because they're not getting the price they want.

It could also mean that the apartment was sold as is the case with another apartment in a building that I'm also buying into. Furthermore, while I've been in contract for about a month, StreetEasy still shows it as an "Active Listing" because the sell side broker hasn't updated their web site yet to show the place as "In Contract". StreetEasy does not source its information directly from database (forget what it's called) that real estate brokers use. They rely on the information that's posted on the various brokers own web sites (Corcoran, Stribling, etc.) which as you can see is often wrong.

But #4, I've noticed that sometimes StreetEasy indicates that a listing is "not available" without there having been (or before there is) any indication of such on the listing broker's website.