Microsoft is showing Microsoft 365 Family plan ads in Office 2021, offering a discount of over 28$ for a 3-month subscription.
The ads started to appear on August 10, with the ad being tagged as a LIMITED OFFER.
Annoyingly the ad uses the same screen space as the message box asking Office users to enable or disable macros, with the button "Enable content" being replaced with a "Redeem Offer" one.