No, it wouldn't.
The UN Charter has nothing relevant.
There are various drug conventions, but IIRC, all of them have denunciation provisions, and in any case there is no basis for the claim that either violation or denunciation of those conventions would result in expulsion from the UN. Violation, rather than denunciation, might result in a suit and adverse ruling in the ICJ by another aggrieved nation. Or it might not.
sure, there won't be a un police action against them, and today i learned that denunciation is an option in the relevant treaty, rather than expulsion. but, there is a good reason (treaty obligations) why countries won't just 'legalise all the things' and have traditionally followed softer approaches.
https://www.independent.co.uk/news/health/united-nations-wor...