I didn't look very hard for deep learning approaches to pitch detection, mainly because I was really interested in chord recognition (which is a far more interesting problem, IMO.) I didn't find any good research here (would love pointers.)
I did though spend a lot of time studying non-DL approaches to pitch-detection, mainly because I wanted better real-time performance for my game Pitchy Ninja (https://pitchy.ninja).
Yeah chord recognition is a much meatier problem for sure. Anything where the pitch signal gets is blended with others is pretty cool. I haven't dived deep into pitch work yet, but this[1] is a fairly solid recent review paper that I followed with good results for tempo stuff.
I did though spend a lot of time studying non-DL approaches to pitch-detection, mainly because I wanted better real-time performance for my game Pitchy Ninja (https://pitchy.ninja).