Currently, with the legacy quest/item that allows collecting Deep Engrams, the game erroneously(?) displays that it needs to be redeemed at the Sonar Station to earn both types of Keys from Ritual Activities. The Sonar Station itself shows and does nothing to this error, but I can't help but feel like the keys specifically are not being earned except through the legacy Season of the Deep quests. Also there are no bounties available at this vendor specifically despite a weekly challenge of completing 8 of them being available on the map.