I noticed tech portals and blogs, at least Polish ones, consistently claim that #email #alias like username+alias@mail.tld is special unique feature available only on #Gmail 
I learned this exists - and started using it - after getting rid of Gmail, on my #Protonmail account. Didn't try it on other services yet but I expect some providers also support it.
And unfortunately I expect many people outside geek/tech circles got to believe it's Gmail-exclusive.
@robryk @madargon That would be bad both for the common uses of the sub-addresses, and for any legitimate cases where they are truly separate. I don't know if the latter is merely hypothetical or not, but for the sake of protecting the former, I hope it is a thing, just to discourage such assumptions.