The dropdown for the spelling variant (lemma language code) on Special:NewLexeme includes both the correct and the outdated language code for Belarusian (Taraškievica orthography):
The backend also allows both of these to be used when actually creating the Lexeme (Test Wikidata: be-tarask test, be-x-old test). This is not good: the language code be-x-old should not be used for new data anymore. (Compare T235505: Rename be-x-old as be-tarask in Wikidata, and see also T284808.)