I don't believe that "a@" and "a+tag@" being treated as the same address is part of any specification, but I could be mistaken about this.

Many mail servers have a setting to configure which character to use; some default (or used to default?) to "-" instead of "+".

I think you're talking about just allowing the symbol. The plus sign is an allowed character in the local part in the spec. But mailservers are not required to treat it specially and apparently Exchange doesn't. That seems to be the thrust of the comment you answered to.

Tangentially, what's impossible to solve is all the developers out there who build input forms that will not accept an address with a "+" in it and flag it as invalid. They just use a regex that looks for alphabets, numbers, underscores, dots and one @ sign (I've also seen forms that won't accept any TLD other than .com).

The @ is a regular email character that Google happens to treat specially. Same with them disregarding the period character.