TOPIC: A stakeholder's communications preference - where is it recorded?

If a particular stakeholder has a particular communications preference, does that preference get recorded in the Communication Management Plan, the Stakeholder Register, or both?

Great question. I would say both. The Stakeholder register would identify certain info about your stakeholders. This info would then be an input to your communication management plan. Then, this would be an input to manage stakeholder engagement. It is somewhat circular.
In short, you may note one stakeholder only likes email in the register, you then add that requirement to the comm plan and then implement it back at stakeholder management.