If your Battlenet launcher was set to English (US) the message started showing up last night. The message didn’t show up at all if the language was set to English (UK). At the time the hover-over text mentioned the end of Season 20, not Season 21, and the text was truncated mid-sentence. Clicking on the link took you to the Season 21 starting blog.
[ EDIT ]
Portugeuse on EU launcher still showing this… https://i.imgur.com/hXWY37Y.jpg