Granted, I have not booked via phone or in person in ages, and I have never tried asking for business class when booking an intra Europe flight post. business class cessation, so I have no experience of what the reply will be.

But as this would be dependent on the reaction and explanation of a specific agent on any given day, neither do you really. They may very well just issue a Plus ticket with no words of advise, or they may warn that it is Plus and on the specific flight no business class is offered.

However that being said, the confirmation email the passenger will receive will not say business class, or indicate that the passenger is flying business class. Admittedly, booking directly in the airport for an immediate subsequent flight, the email would not be read in advance