There have been some mentions of this over the years and also mentions that some have been told it does not exist.
One specific mention I could find is dated
https://onemileatatime.com/someone-d...-travel-plans/ While AA related, in the comments there is a UA case
Concern if will this block online cancellations
Google search the number?
Did you ever call a "United Reservation" number found by a google search before this, some are not United and can be a problem.