You can throw grenades straight through doors and destroy items inside, I have only tested with a wooden door so far.
I can confirm it works with metal doors also. Me and my group were trying to raid a base, and used 20 grenades on one door. The door didn’t budge, or become damaged.
Works with metal doors as well, it’s been around for a while, thing is it takes so many grenades to destroy anything that it’s not really even worth anyone abusing. I assumed Garry would already know about it, but email Garry@Playrust.com if you think it’s needed.