straight ahead jazz releases from the 60’s and 70’s, they sound natural and I get a wonderful sense that I am there at an actual performance. Not so much with releases produced in the past couple decades.
I am not an expert for mixing aesthetics of jazz, but my assumption would be that the general philosophy of a sound ideal in this genre has changed rather away from your taste. As you write ´natural´ and ´I am there at an actual performance´, well that is indeed not what the vast majority of jazz records are aiming for, in my understanding.
To come somewhat closer to the ´feeling of being there´ with two channel stereo, the mixing engineer would have to add a lot of reverb or place main microphones much further away from musicians towards the diffuse field in the studio, which is heavily compromising the feeling of rhythm, drum kicks, directness, clean, tight, low bass and groove, if that makes sense.
I would assume that the inevitable compromise of stereo recordings and different philosophies how to deal with it, might be the main reason for your observation.
When it comes to ´natural´ sound aesthetics, I would rather see such at play with classical recordings, and in this field sound quality has vastly improved the years after proper recording equipment was made widely available. Would say in the laste 30 years the majority of recordings is very good or excellent.
And many of those sound objectionably harsh, as if they contain an abundance of odd-order harmonics or IMD.
Have never heard of many jazz recordings sounding harsh. Tbh I don't know any examples of technically flawless recordings of natural instruments which I would file under ´harsh´. This sounds to me like a very strong indication something is wrong with your listening equipment, either loudspeakers, headphones or the room in which you are listening.
Could you give an example for a harsh modern jazz recordings please?
Is this due to an over-reliance on electronic instruments, drum machines, etc? Poor/overzealous post-production processing?
Are you talking about electronic music and popular music? Well, aesthetics in this field have changed over time. Maybe you simply don't like what the majority of listeners likes. It is also very much depending on the actual genre. Some have moved into a more artificial sound idea (like Hiphop, R´n´B and some subgenera of rock), some not.
There was a period when many popular genres were obviously aiming to sound more aggressive, dense and not clean, involving over-compression, distortion and effects like exciters being used extensively. I am talking about the mid-1990s to maybe around 2010, a period from which some recordings don't meet my expectations. In the last 15 years, things got better, and a lot of pop, electronic and rock recordings sound surprisingly enjoyable even if involving a lot of effects. My explanation would be that with the dominance of in-ear monitors, people walked away from this dense, aggressive ideal.