I doubt "pure" is the best word to describe 清楚, but it does mean something along the lines of "simple", "innocent", "clean", "plain", "tidy", and so on. Please refer to that YouTube video or Google Image Search for the connotation of the word. As a matter of fact, 清楚 is a common word that virtually every teenager understands, and it has been around at least for 100 years without any significant change in the meaning. I'm curious what page said this term is VTuber-derived. Recently, this word may be especially common as a category of the faces and personalities of young female people (including idols and VTubers), but it is not used in a different way from its original meaning.
EDIT: As pointed out in the comment section, the word 清楚 seems to be often used sarcastically in the VTuber community, and the word seems to have been imported to fans in the West along with the incorrect definition. See this article. This is a kind of local joke that only works in the VTuber community.
「清楚」の本来の意味は「飾りけがなく、清らかなこと」。今日では、慎ましく清潔感のある美しい女性を表す言葉として使われている。
Vtuber界隈においては、公式設定や自身の主張で「清楚」を標榜していてもおよそ清楚とは言いがたい言動がしばしば見られ、そうしたVtuberの形骸化した「清楚」設定は本来的な用法とは正反対とも言える新しい語義として受け入れられている。