Mail and Plate Base Items used to craft Shadowlands Legendaries have Mythic Sanctum of Domination set appearances. With grey and white gear becoming available for transmog in the update, you can use them for transmog to learn the set appearances.
This will only work with Plate and Mail Base Items, however (thanks to bartek8003 for clarifying). If you have a few spare Base Items in your bank do not delete them! You won’t be able to restore them.
Image courtesy of Reddit user art_prominence.