As noted in #1725, this note has lost its meaning with recent changes to the URL standard:

Some language specifications reference Requests For Comments that preceded [URL], in which case the earlier RFC applies for content in that language.

This used to be a note about RFC3987, in which case it was a caution that you can't use internationalized IRIs if only RFC3986 or earlier are allowed by a format.

I suppose this is still true -- just because we talk using URL standard language doesn't make the URL standard applicable to other formats that don't reference it. But perhaps we can make this clearer and also, as @iherman noted, find a better position for it so we can make it clear it applies to the whole section.