Is that a feature that you can count on working indefinitely?

To the extent that Google supports standards, yes.

RFC2822 allows A-Z,a-z (case insensitive, 0-9, the chars ! # $ % & ' * + - / = ? ^ _ ` { | } ~ and . (assuming the period is not the first or last char) Any server that rejects an email address for containing one or more of the above characters is in violation of RFC2822, and should be considered broken.