I think they should have made it clear that the cards are leaving - I only vaguely remember 1 notification in-game. They do this pretty well with other events like the Fire Festival and special Arenas.
If you follow updates outside of the game, you might already know that the cards would be leaving 5 December when the new patch hits. This was also on the forums: Wild Cards rotating out of Standard on Dec. 5
However, no mention on PlayHearthstone news beforehand, and some mentions on Twitter and Reddit instead.
https://youre.outof.cards/media/uploads/ee/7a/ee7a6e47-bf02-4e52-853a-50012bc12f0c/hearthstone_screenshot_10-08-19_130531.png