I definitely think it has to do with the 50th - whether it has to do with what you said and new procedure /benefit to those staying on property or they just don't want all the parks already booked up for the 50th now or many other reasons I think they are stopping it before the 50th. I think that might be the end of their fiscal quarter/year too or somewhere around Sept 26th so could be another reason
As someone who has a stay scheduled to start Sept 29th, 2021 I selfishly I could book park days already and have that locked in