I just confirmed that yesterday, I successfully registered for the Platinum Challenge.^

fwiw, the reason no email confirmation nor online progress tracker is provided is that the Platinum Challenge is considered an account "exception"?
Still, imo, it's absurd that it we are unable to refer to the written T&Cs.

For example, yesterday after registering, I asked the agent "What, if any brands or properties are not considered eligible as a Challenge stay/night?" That agent, stated that only "Marriott Executive Apartments" don't count towards the Challenge.

Today, the agent that confirmed my registration, rattled off a bunch of brands and individual properties that do not count towards the Challenge.

I couldn't type fast enough to capture every place mentioned, but those I did catch include, the Cosmopolitan (LAS), Atlantis (Bahamas), Ritz (SIN), Ritz (HNL) and many other Ritz properties, anything in the Protea Brand, the Maxwell (NYC), Residence Inn Mayo Clinic (JAX), Marriott Conference Centers (Oklahoma and somewhere else?). Bottom, line verify before you stay!

I also asked if paid nights at Vacation Clubs like the sister properties in St. Kitts or Aruba, also count towards the Challenge? She saw nothing that excluded those properties but I'd appreciate any data points from other Challenge participants.

fwiw, the agent also suggested that when staying at any non-US property, to insist the hotel give you a printed folio (rather than a promised email). Marriott needs proof of paid stay, as their system does not track and can't confirm non-US stays.