True, under the right conditions, leather will absorb some moisture and that can be damaging to a gun stored in contact with it. In addition, the chemicals with which leather is tanned can be corrosive to metal that is kept in contact with it. That being said, holsters made by the better brand holstermakers are made of leather that is tanned with chemicals and methods that reduce the corrositivity of the leather. Personally, I do not keep any of my guns in leather.
I had a custom made knife that I was very proud of and I had a saddlemaker make me a scabbard for it. He used the same leather that he used for making saddles. I put my pretty knife in the scabbard and put it away for a few weeks. When deer season got here, I went to take it out and check to be sure that it was sharp, much to may dismay, I found it covered with a layer of rust. After a LOT of work, I was able to restore it to its former beauty but I learned my lesson.