Mentioned three (3) times in the forum guidelines:
The sending of threatening Private Messages (don't do it);
Please keep personal conversations in the Forums Private Messenger;
Trolling behavior includes...sending (a particular member, employee, moderator, or administrator) rude or unwanted/unsolicited private messages.
I haven't been able to find anything else.
Edited to add: Well, I guess for the sake of completeness, the announcement of disabling them: https://forums.xfinity.com/t5/Customer-Service-Archive/Private-Messaging/m-p/2936832
Thanks for the reply, WTP. The reason for the question was the image posted below. Is this bogus,or is someone messing around with the integrity of my Private Messages. I saw no info within the PM to delete old messages. In all my years of using the messenger, and folks commenting within these forums, I've never seen mention of any number limit.
Yep, it's happened before Notice the thread was locked with no resolution or explanation.