The Shade type is a bit of a weird type. Unlike the Dark type and the Shadow type, this type is actually related 100% to darkness. It is also technically the Void type of this game. The Shade type also includes nocturnal Pokémon such as Noctowl. Generally, this type is the opposite of Light in terms of elements.
It was originally named Night but it got renamed due to an existing Night color.
This type was created due to Dark being more of an evil type (in fact, its Japanese name is literally "Evil").
Even with the symbol depicting a moon (easier to see in Pokémon Masters), it's still mainly evil, with Guzzlord, Yvetal, and Krookodile being some examples.