Is this a per-contact setting or a "universal" one?

It's a universal setting. You have to enable it per third-party app, though. You get to choose whether you want to see them listed with WhatsApp chats or in a separate folder

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odo1242 7 hours ago | parent | prev | next [-]

Account-wide. Though you can only turn it on in Europe.

	▲	benj111 7 hours ago \| parent [-]
		When you say Europe you mean the EU? I'm not seeing an option in the UK. (Yay Brexit)

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dfajgljsldkjag 7 hours ago | parent | prev [-]

Each whatsapp user needs to enable the setting once to allow chats with multiple number of third party users.