Ancient Plates are a special class of rare items, based on the 17 Arceus Plates from the main series of video games. On GPX+, their sole function is allowing the user to embark on the Death of the World Exploration. This exploration rewards the user an Arceus Egg of the type corresponding to the plate that was used to embark on the exploration. A user can only use a specific Ancient Plate to embark on the Death of the World exploration one time.
Ancient Plates are obtained through three different methods. The first is no longer accessible, but users who hatched a shiny Buneary, Furfrou, and/or Honedge during the March 2015 Site-Wide Shiny Hunt, a random Ancient Plate would be received. Otherwise, Shards can be obtained through hatching a Shiny Pokémon (although this can only be done once per day, from reset to reset) and placing 1st in the type-respective Poké Hunt, after obtaining all five Cosplay Giftboxes. After obtaining five of the same-type Shard, they may be forged into an Ancient Plate.
By nature of the way Ancient Plates work, it is impossible for users to obtain a Normal-type Arceus other than as a reward for certain special events.