Seems like I was also given the option for this Feature. It only seems to work on a few properties and available on Member or Standard Rates on Base Rooms.
https://imgur.com/NKwpnnL
https://imgur.com/0ewRBTv
https://imgur.com/9LQMsoC

Since I'm an Explorist, resort/destination fees aren't waived for the cash stays. Any combination of nights that have a cash paid night will still have the fee charged, which is correct. The award nights don't have such fee. For example, if you use Pay My Way feature with 2 Award Nights and 1 Cash Night, you only pay the taxes and fees on the 1 Cash Night.