Just as the title suggests, I didn’t receive a free deck as a returning player, nor have I gotten the chain quest for not logging in for 90 days.
I’m not familiar with the requirements and conditions for returning player rewards.
Do you have an official blizzard link that explains things?
My understanding of this feature is that you should see a special page with the available “borrowed decks” when you start a match, for a week after first logging on after a 90-day absence. Then, when that weak has passed, you should be given the option to select one of them and have it added to your collection.