With Core in pruned mode, is there a command to get the hash of the oldest stored block?
I don't think so. It would be an interesting command to have. If there isn't, and until then, you can manually check it by subtracting N from current last block height, N being the number that you entered in the "prune=N" setting in your bitcoin.conf
So do:
getblockcountThis will give you the latest block you have. For instance for me right now is 526788, so let's say your prune N value is 550
526788-550=526238
getblockhash 526238And there you go. If you try to get the hash of a block that is behind N you should get an error, something like "block not available (pruned data)".
A cvar that automates these steps could be added I guess.