Drop u:locale

[eemeli]

• Closes [FEEDBACK] u:locale may be problematic for ICU4X and interoperability #976
• See also Drop the u:locale option #1014
• See also 2025-02-17 meeting notes

We should figure out what to actually do about u:locale, rather than just leaving it as Draft (as was done in February). My position is tending towards the feature not being sufficiently well motivated for the added complexity that it introduces. I'm relatively certain that the rare cases when it's actually needed it doesn't need a solution baked into the spec; this is also discussed on #976.

[aphillips]

I obviously disagree with the "lack of motivation" argument. I get the argument from the ICU4X folks that it would be hard(er) for them to implement. But I have a reasonable amount of experience needing it for uses other than I18N demos (usually it is needed for tailoring locale options). Implementations that choose to implement this optional feature can make good use of it.

That said, leaving it in draft limbo is not helpful. It's either well-described and reserved for the specific use or we let implementations create namespaced versions for their own use. Probably better to do the latter if this won't be universally portable anyway.

[aphillips]

Changes look complete.

[zbraniecki]

I'm relatively comfortable with the assumption that I will be able to make it work in ICU4X, but it will require a double-pass approach which would be quite unfortunate.