I was wondering if there would be runs for Switch Online Expansion Pack as a platform, as it isn't VC or an Emulator.
They're platforms but they don't have a category.

SwitchVC is the platform for NSO. It's all part of the VC category (WiiVC/WiiUVC/SwitchVC).

As Retro said, Switch Online runs can be submitted under the VC category. Just choose NSO as the platform when submitting as well.