David Hopwood said: > Note that if deprecation implies no longer treating these characters > as ignorables,
It would not. The only character *property* implication that deprecation of Plane 14 language tags (or any other characters) would have is the requirement that they gain the "Deprecated" property. (See PropList.txt in the Unicode Character Database.) > then that causes new software that sees existing data using > plane 14 tags to break (to some extent; probably not fatally). OTOH, if > deprecation does not imply treating plane 14 tags as ignorables, then > nothing is gained: the complexity of filtering is still there, but the > characters can't actually be used. Deprecation in the Unicode Standard does not mean that characters cannot actually be used. In fact, many generic implementations, such as low-level libraries which report character properties, will continue to implement them, precisely because higher-level processes will need to know that the code points in question *are* deprecated (along with whatever other properties they may have). What deprecation in the Unicode Standard means, basically, is that a particular character or set of characters is noted as a horrible encoding mistake, and that any implementer in their right mind would look to use the suggested alternatives as a better way to approach whatever misguided goal the deprecated characters were originally intended to achieve. As Asmus put it: "Since we can't remove them, we would deprecate them, so that countless legions of implementers can forget worrying about a feature deemed desirable but never put into practice." --Ken P.S. I have to agree with John Hudson, Asmus, and others that the issue is not about the usefulness of language tagging per se, but whether Plane 14 language tag characters themselves, as currently defined, are an appropriate mechanism for indicating language tags in Unicode (supposedly) plain text. Doug's contribution would be more convincing if it dropped away the irrelevancies about whether the *function* of language tagging is useful and focussed completely on the appropriateness of this *particular* set of characters on Plane 14 as opposed to any other means of conveying the same distinctions.
