I look at stats (disk space usage, monthly bandwidth transfer) on the root cPanel page monthly. But I got the same letter, and when clicked the link in the letter also was told "your username could not be found in the list of accounts suspended for being inactive". After that I went to cPanel one more time.
Ads Standard plan in my case.
P.S. Current Style: [root]
(I never changed cPanel skin), language English. Each time I visit cPlanel it asks for basic authentication (username, password), it's normal.