Most modern leathersmiths, or at least the better ones, use vegetable tanned cowhide (steerhide, actually).
In elder days, cheap holsters were made from chrome-tanned leather, which left salts in the hide, and these would react with the blueing on guns if exposed to moisture.
None of the holsters obtained from Lou Alessi, Matt Del Fatti, Ken Null, Milt Sparks Leather, or any other "custom" maker you can readily name will have this problem, as they all use vegetable-tanned leathers.
Guns neglected and left in these holsters can still rust, just as they might if exposed to moisture when not left in holsters, and of course the holster leather can hold moisture next to the gun at points of contact, but I've personally left guns holstered in leather holsters for months at a time in a low-humidity environment (inside my safe) with nary a spot of rust appearing.
YMMV, etc.