That's interesting. I sent an e-mail to the team that administrates the opt-out scheme on 7th April and got the following response from an 'Advanced Customer Advisor' named Enya:

I have had a look into your enquiry and I believe you will not receive the EU Referendum Leaflet as this item is not classed as an exceptional circumstance. A special circumstance would be something like a missing persons leaflet or something to do with your health and safety.

The response doesn't sound too confident (I believe… implies she isn't quite sure) so it may well be the case that government has insisted that the leaflet is delivered to every household, including those that are opted out. My guess, though, is that nobody really knows and that different people with Royal Mail are making up different stories.

If you want to follow this up with Royal Mail, the reference of my query is 1-3540519579.