Testing of the button is currently underway. It is likely to replace the secondary Windows key.
Microsoft is considering adding an Office key to keyboards. The new key is intended to facilitate quick sharing of documents and files online. This is reported by The Verge.
Microsoft plans for the key to replace the secondary Windows key on the right side of the keyboard or the dedicated menu key.
Currently, as discovered by WalkingCat, the company is conducting a survey among testers of the Office key and receiving feedback on how it works. It is likely that Microsoft is testing the concept with the latest Windows 10 May 2019 update.