The + thing is common but is absolutely not universal. I can alias individual + emails on my system just fine (which means that x+a@example.com and x+b@example.

The + thing is common but is absolutely not universal. I can alias individual + emails on my system just fine (which means that x+a@example.com and x+b@example.com are not equivalent) and most smtp servers allow you to set an alternate character (I set it to '.' and '_' so x.a@example.com and x_b@example.com are aliased together). In short, Netflix cannot assume things after the '+' are irrelevant and Netflix absolutely cannot canonicalize random emails like that.