They need to be setup by the homeserver, i.e by the admin of the "instance".

Do you mean exclusive/tiered access to certain rooms only for subscribers?

If you are telling me that such a thing doesn't exist for Matrix already, you just gave me a perfect thing to offer on communick

I don’t know about other platforms, but YouTube membership is totally implementable on any other platform.

The workflow anyone need to implement is the same flow Discord has implemented:

1. Perform OAuth to get the user’s own channel using the mine filter on channels.list end point. This way the service can know SomeOneWatching is owner of channel UC1234ABCD
2. Perform OAuth to get the host’s members on a fixed interval to get a list of all members, and match it against all known users’ channel IDs or target individual user like SomeOneWatching’s UC1234ABCD channel ID as part of filterByMemberChannelId on the same members.list end point.
3. Upgrade users’ groups on the service to reflect membership accordingly, no direct YouTube partnership required.
4. Revisit the same flow in 2 regularly to downgrade when memberships are not renewed; beyond the pubsubhubbub which notifies subscription content updates (new uploads/deletions) on a subscribed channel, YouTube does not have a push notification for automatic updates. This is why there’s always a slight delay when membership status changes.

Source: I’ve worked in YouTube adjacent company using all of their public and several proprietary APIs for around 10 years now. I’m fairly familiar with their API offerings.