Oh, yikes: when did the + stop working for Apple support? That's going to bite me at some point. (And in general, yeah: I've had multiple cases over the years where the "+" only caused problems after signup. One of several reasons that I eventually stopped using those forms.)
And just to put it out there: "+" addresses aren't specifically a Gmail thing, they've been supported by most(?) mail servers for a very long time.
I think it stopped working about 3 years ago for me. I was in Japan for 10 years and moved last year, so I have been using a +japan address with apple. Of course in that time I've been using icloud and iphotos so I changed the region to my current country rather than make yet another account. I also removed the "non+" gmail address from a very old apple id thinking I could then attach that gmail address. Apple told me it would take a few months for the system to let me.
It's been a nightmare. Apple lets account creation succeed without email verification and with SMS verification. So with my very common name, as soon Apple allows it, someone with a similar name uses my email to make an apple id, preventing me from using it. Without knowing or owning their phone number, it's impossible for me to fix. It takes 2-3 weeks and multiple hour long phone calls with apple to free the email again, at which point someone else uses it and the process repeats. Every phone call is horrible, it is really confusing to explain the problem. Every phone call starts with the support staff believing I am typo'ing my email address. After a few times I gave up trying to fix it.
And just to put it out there: "+" addresses aren't specifically a Gmail thing, they've been supported by most(?) mail servers for a very long time.