In legacy I always built my houses as close to rock as possible to get a safer side. If anyone wanted to get my stuff he had to bomb all the way through my house to the part they could not reach from outside (if they did not glitch into the rock and blow up the one wall on the backside).
With caves now we could build hobbit-like houses or even put an entire house in front of the cave so you have to go through the entire house to enter. But as I once noticed with a sack of precious stuff I hid in a bush behind my house: decay is a sonofafemaledog…
I seem to recall they even added that boxes outside a house will decay even if you open and close them daily (not 100% sure about that one though)
Decaytimer got reset for all items that are connected to a foundation that got updated by adding structure or opening/closing a door. So will everything in my homely cave decay because it touches the ground?
A workaround would be that by putting a doorway at the entrance of the cave it would become a “house” with the same rules for decay.
But I am not sure if a cave counts as a placed object or is a second layer on top of the terrainlayer the map is made of.