Here were the 2019 participating brands:

Alamo Rent-a-Car
Avis Car Rental
Best Western International
Caesars Entertainment
Choice Hotels International
Expedia
Accor Hotels
Hertz Rent-a-Car
Hilton Hotels and Resorts
Hyatt Hotels and Resorts
InterContinental Hotel Group
Las Vegas Sands Corporation
Loews Hotels and Resorts
Marriott International
MGM Resorts International
Omni Resorts and Hotels
SeaWorld Parks and Entertainment
Universal Parks and Resorts
Wyndham Hotels & Resorts

I assume most of those were lined up for the cancelled 2020 promotion.

Since the offer previews don’t seem set to show up until October 4th, and as the sales go live on October 7th, it seems to me that they are scrambling for participating companies and/or to get staff to work on having everything ready to go for this 2021 promotion. Probably hit with extraordinary marketing and tech resource constraints due to changes that hit during and from the pandemic — not only from the promotion runners but also their promotion partners.

Note that the previous preview offers date was showing as September 27th.