Hunter’s Reward is a divination card. A set of three can be exchanged for The Taming.
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Hunter’s Reward can be dropped in the following areas:
Grotto Map • The Putrid Cloister Map• Underground River Map
GGG have never (or almost never) commented on drop rates or locations of items. All I can say is that the card is still obtainable in the Delve league. In what way I do not know.
The wiki page indicates that it drops from Grotto, Putrid Cloister and Underground River. At a price of 60 chaos per card you can probably expect to do several tens if not hundreds of cards before seeing a single card drop.
I can’t really provide any more information than that (although it would be extremely surprising to me if Aesthete didn’t drop from regular old Shavronne, given that it isn’t that strong of a card). But you should be aware that the drop rate of Hunter’s Reward in particular is going to be extremely low, given that it gives a megarare item and only requires 3 cards.
Also, GGG will pretty much never confirm or deny anything about items that isn’t technical in nature. They prefer to let the players figure stuff out themselves, and are fine with people providing misinformation.