amiibo functionality was added in a December 2019 update.
These remixes can be unlocked normally through gameplay; scanning an amiibo unlocks its corresponding remix immediately.
Shovel Knight (normal and gold) gives “Strike the Earth!”
Plague Knight gives “Flowers of Antimony”
Specter Knights gives “La Danse Macabre”
King Knight gives “In the Halls of the Usurper”
Source: ThatLostGamer on YouTube (video) / Thanks: Robin64