Imo, That "sheen" of older records besides being mixed by great engineers
also comes from the analog outboard equipment that was used in the recording studio.
I don't think even today most pro tracks are done strictly all ITB.
And I think that emulations of outboard analog equipment are just that.
Not saying some of them don't sound great, or that nobody's ever gotten a great mix solely ITB.
I just think that's why some of this old outboard stuff is hot again and selling for big bucks.
I also think the choice and use of reverb has much to do with that sheen.

Not that I've personally achieved that sheen on my own
but from what I've heard and seen - just saying...