Eavesdropping through walls is something that has always skirted along the lines meta-gaming and power gaming. This has always been something that has been a grey area within the community.
It is common sense not to listen to and use information gained through
However, listening through areas such as clear glass (that both sides can see through) and wooden doors are perfectly fine.
It’s common sense, which is why there isn’t even a rule for this in the first place.