You said this happened "a month or so ago" from this letter dated 11/20. That makes it approzimately 10/20. FWIW, I don't understand why you didn't post the exact date this event happened, because you must know it and it is important to your complaint.

Now, in a reply below dated 11/21, you said that you received the first written notice regarding your credit limit reduction on the same day (11/21).

This means it took a month to tell you your credit limit was reduced. This seems unreasonable, unless something was lost in the mail. Have you called and discussed the fact you received no notice for a month?