be-x-old.wikipedia.org was renamed to be-tarask.wikipedia.org (T11823).
Now, querying the API with langlinks for be-tarask doesn't work, but querying for be-x-old does work.
See:
* https://en.wikipedia.org/w/api.php?action=query&prop=langlinks&format=json&lllang=be-tarask&lllimit=1&titles=Italy -> returns nothing useful.
* https://en.wikipedia.org/w/api.php?action=query&prop=langlinks&format=json&lllang=be-x-old&lllimit=1&titles=Italy -> returns the correct target
I'd expect both to work: be-tarask as the current and be-x-old for backwards compatibility.
I'm not sure whether it's a bug in core, in Wikimedia configuration, in Wikidata, or elsewhere. This might be related to T111822, but I'm creating this bug in case it isn't.
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**__Specification:__**
* [ ] One-ILL-per-language
* [ ] Do not replace ILL prefixes to ensure correct URL based on interwiki / interlanguage table (`interwiki` `iw_prefix` field)
* [ ] Allow multiple ILL prefixes for same ILL lang code
** [ ] `InterlanguageLinkCodeMap`
* [ ] Allow treat deprecated language code as different ILL language, especially for BCP-47-overlapped (`ExtraInterlanguageLinkPrefixes`)
* [ ] Make API lang parameters case-insensitive
** [ ] Decide whether to support BCP 47 language code for API lang parameters
* [ ] Output both prefix and BCP 47 language code for API queries
** [ ] Decide whether to output MediaWiki internal language code for API queries