Library producer here (echo collective, echo collective:fields)
regarding cleaning background sounds -
All background fx cleaning is destructive and alters the source in some way. The reason this step is not done is because that type of work really requires context to be done well.
In other words, if you place a sound with some background wind or traffic into a mix that has elements which mask it, you'll never hear the wind or traffic anyway, so why degrade the sound by cleaning out parts that won't rate in the mix? Of course, there's always a balance, and if a sound is actually ruined by a background sound then it should not be included in the release, but that said, its amazing what you can get away with regarding low level noise when cutting sfx into films and games. And if you do need to RX something, you can use the context you're placing it in to inform exactly how much NR you need to do.
As an example, on our utility helicopters library there was a highway about a mile away from the tarmac. We obviously didn't use takes where a bomber was cruising overhead, but we were similarly unwilling to discard a take because of a small amount of traffic noise present in the quietest parts of the recordings. With that said, any attempt to completely isloate the helicopter rotor sounds from the traffic would have had at least some impact on the recordings so the mastering process only included time aligning the various mics, hipassing judiciously on certain channels, level balancing, and output. No multiband expansion, no algorithmic noise reduction. There were some spots where I went in with RX and removed various bird chirps and other tiny blemishes, but that was the extent of it.
now lets talk about editing tails - in our pro hockey ambiences library the methodology was this: arrive early, set up mics in one location, record entire game, teardown. change locations in the arena and repeat.
For the various crowd swells and cheers my heads and tails were dictated by the moments that the music director (DJ Whiz) would play a song. Since I can't include copyrighted music in my recordings (and since music ruins crowd recordings regardless of copyright issues) my decision was to end the file (with no fade) the moment before the music started playing. Sometimes Whiz would be right on top of a play with a music cut, and I wouldn't have much for crowd tails. Sometimes he'd let it breathe or otherwise search for a different cut for a bit and I'd have nice long tails. In either case, I left max tails in the recordings and cut them off with no fade (ok, a 5ms fade) right before the music kicked into the recording.
The reason for not including a fade is the same reason I don't do NR on library releases - I don't have context. I'm giving you all of the clean sound that was in the air in that moment, and relying on you to use that sound as you see fit in your project. If I were to do a fade that I might like, you'd be cursing me for that the moment my fade didn't match your context.
field recording is a messy business. The world is polluted with noise, and while good mic choices and technique will yield very good recordings, they won't stop the world from being noisy.
With that said, the industry standard is to do zero noise reduction or expansion, and to leave tails as long and unfaded as the recordings allow. Its not a bug, its a feature. :)
tldr - noise reduction and fades are destructive and require context.
