No, that only shows that the ISO 3166-2 registry is a bad basis for Unicode flags since having things lose meaning over time should not be acceptable for a text encoding.
Flags have another issue here in that they can change even when the country stays the same - a recent example here being Afghanistan, but also France who recently changed the official shades of the colors in their flag. Ideally you'd want a new Unicode representation for any changed flags in order to not retroactively change the meaning in old documents.
It's up to the installed fonts really. I don't know if the combination of S + U is standardized as a Soviet Union flag emoji, but even if it is, your locally installed fonts may not contain every single flag emoji, so the browser would still fall back to rendering the two letters instead.
