Hmm… it didn’t say that on my launcher until I changed the language from English (UK) to English (US)… and whilst the main text says Season 21 ending soon, if you hover the mouse over it it shows the text
Diablo 3’s Season 20 is winding down, and a new season is upon us. That means it’s time to start wrapping up your Seasonal goals and capture
Yeah, I didn’t truncate the text, that’s just what appears. Clicking on the splash takes me to… https://us.diablo3.com/en-us/blog/23428368/season-21-trials-of-tempests-is-now-live-6-4-2020?blzcmp=app …which is just the Season 21 starting announcement blog.
So, not only is the announcement not available in all languages of the launcher, the hover-over text is truncated and the link points to an article that doesn’t talk about the end of season.