Notifications by space

If I turn on server email notifications, but want a certain space to not send any notifications, what is the setup for that? Do I just switch it to "other recipients" and then keep the list empty?

Good question. Try switching to "other recipients" but specify no email address, or specify a non-existing address.

We should make an option for that.

Somewhat related follow up. Is the Default Worker account the account used to connect to the email connector to send the notification emails?

Yes, that's correct.

Little more clarification.. I just tested and it looks like when using an imported Exchange Online Oath connector, it uses the account that authorized the connector. Does that sound right?

Sending notifications with an imported connector works exactly as if you sent an email using the "Send email" action - i.e. with the credentials or OAuth tokens stored in the connector. The Server doesn't modify the connector or re-authorizes it after importing.

Sending notifications is technically done by the Server service which is the same process as the Default worker.