The + thing is common but is absolutely not universal. I can alias individual + emails on my system just fine (which means that [email protected] and [email protected] are not equivalent) and most smtp servers allow you to set an alternate character (I set it to '.' and '_' so [email protected] and [email protected] are aliased together).

In short, Netflix cannot assume things after the '+' are irrelevant and Netflix absolutely cannot canonicalize random emails like that.