Solved or solvable? IME, sibilance or better yet, negative sibilance, has been with us from the beginning and is one of the effects of universally noisy AC. I attest that negative sibilance is nearly or entirely solvable by superior forms of AC treatment where superior means everything since inferior forms would most likely add even more negative sibilance. In my case, I use superior passive dedicated little line conditioners at each component.
But negative sibilance is a bit tricky in that playback electronics generally induce negative sibilance into the mix. This is solvable. But then there's negative sibilance induced at the recording mic / recording side of the house and embedded into the recording. This is unsolvable during playback. And since it's in the recording, you actually want to hear this negative sibilance.
Many a recording will have a mixture of both negative sibilance induced for sure during playback since AC noise is universal and few employ superior AC treatments but also those induced during the recording.
So how can you tell if your superior AC treatments work to minimize or eliminate negative sibilance induced during playback? Good question. If you hear a female vocalist pronounce the "ssssssss" sound in a clean, clear, and precise ssssssss at least once you should rest easy that the problem within your scope (the playback side) has been solved. Even if you hear other times in same recording when it should be a precise "ssssssss" sound but instead you hear an "shhhhhhhsh" sound. One need only to fly to the moon once to prove it can be done, right?
The nasal part of your question, I'm less experienced with but I have heard it a few times in others' systems.