In 2016, players were given a minecon cape pack. This could be used using a skin pack, still can be. In 2020, players were given founders cape but this also added a new capes section and now you could use any skin with a cape. This causes an issue though. 2016 capes didn't appear there; thus, can't be used on a custom skin.

Add 2016 capes to the new sections as they are an exclusivity to old players. Thank you Mojang! Very cool!