I wish it was just edge cases! I've bumped into so many services that can't handle "+" in email addresses, some seem to be on purpose (presumably to prevent signing up multiple accounts to the same address) but others are just old school bugs. My fav is services that send the email address to an API including the email address as a URL parameter, but forget to escape the email address so the + gets butchered in the process.
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.
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.