I found the post. It says, some guy on Facebook said that he heard,
"Dealers that have large allocation numbers are going to “lose” 16-20% of their 9 month total allocation number for 2020 Model Year production!"
This doesn't mean that production will be reduced. They may simply spread the love to more dealerships than a select few.
Sounds like more "he said, she said".
I choose to not believe anything anyone says unless it came directly from GM in an official statement.
If a dealer says it, it's a rumor. If a GM exec says it, it's still just a rumor.