Are devices added through a linked service like Kasa, SwitchBot, Ecobee associated with the hub and count towards the device limit or associated with the app and don’t count towards device limit?
They are associated with the account, not the hub, but they still count towards the 200 device limit. The limit doesn’t care what the protocol is. If it has a device tile in the app, it’s counted towards the limit.
People who don’t have a hub at all can still hit the device limit.