Is that block a recent thing? It might be different as for my on GSuite account I can add the name+addon@mydomain.com - it might just be a difference between the "public" Gmail system vs the Gsuite Gmail system. In which case, my question is completely invalid and feel free to ignore ;)
Note: This comes from my own developer account having 3 name+addon@ accounts live, and working with things like ApplePay etc for testing.
Developer with a name+addon@gmail.com Apple ID here. No idea what OP is talking about, I was able to generate my Apple ID and sign up for the dev console no problem.
Apple now strips the +... part when you create an ID. So if name@gmail.com is already in use, name+addon@gmail.com will be refused with the message that name@gmail.com is already an Apple ID.
Note: This comes from my own developer account having 3 name+addon@ accounts live, and working with things like ApplePay etc for testing.