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.