In math, Grothendieck's correspondences are quite famous. Here's a book that is entirely 300 pages of letters between him and J.P. Serre

https://webusers.imj-prg.fr/~leila.schneps/grothendieckcircl...

There are many others for Grothendieck though.

Another example is how Godel wrote a letter to Von Neumann towards the end of his life. This letter contained, among other things, the (now very funny) question of whether a certain NP complete problem may be solvable in quadratic time.

https://www.cs.cmu.edu/~odonnell/15455-s17/hartmanis-on-gode...

Practically though, modern correspondence is often through a disjoint set of technologies, that (importantly) someone cleaning up the estate of a deceased person does not necessarily have access to. So it seems unlikely we'll get this kind of insight going forward (with notable exceptions, for example the Epstein emails).