The latest LDML spec says (tr35.html#Language_Tag_to_Locale_Identifier)
If the tag is one of the five deprecated grandfathered tags (cel-gaulish, i-default, i-enochian, i-mingo, zh-min) remaining after step #1, prefix by "und-x-".
However, ICU has had replacements with somewhat "better" language subtags:
I think I came up with the ICU tags years ago, and the language choices would be better than "und". Maybe a good idea to change LDML (even though these are edge-edge cases).
I suggest we use these in the LDML spec and data.