Appeal to ISO 639 RA in support of Elfdalian

[Shawn Steele]

> Retiring subtags is easy, but getting users to migrate existing data and implementations to the new subtag is hard.

That's what I meant.  In practice, when we switch codes, such as when the industry has now standardized on a different variation, our customers experience a lot of pain during the transition.  The procedural easiness almost aggravates that problem!

-Shawn