I never heard of the Platinum Challenge and do sign up for all the promotions. I'm a lifetime Gold member (nearly 40 years a member) and since Oct 1, have stayed 14 paid nights. I am 12 short of the 50 required to get platinum and I expect to pick up about 10-11 before the year is over. I contacted Bonvoy last week and asked if they would make a one-time exception if I get those couple of nights just after the end of the year. They just quoted the standard rules (must have 50 by Dec 31) and in my reply, I said that I supposed that meant no. Their reply made it clear that it was indeed no. That's when they first mentioned the Platinum Challenge (they said to sign up in 2020 never mentioning anything about 2019) and so I replied stating that since October 1, I have stayed 14 paid nights at 11 different places and asked if those would count. I am awaiting their reply but expect them to decline to allow it since I didn't sign up for the Platinum Challenge ahead of time (as if I wouldn't, knowing how many nights I had coming between Oct and December).

Update:
I never got a reply to my last email so I called. The agent said that I could sign up now (the email only talked about signing up in 2020) but declined to allow me the nights since October 1 even though he had no explanation as to why I was not notified nor why the email didn't suggest that I could sign up now.
On the other hand, he did thank me for my loyalty. I wish I could have said the same about the company's loyalty to me. I'll remember that every time they mention loyalty at hotels. It only goes one way.