They have replied to me. Seems that Slack is no longer selling in China mainland, neither by SFDC nor by Alibaba Cloud. However if your are a Hong Kong entity,

They have replied to me. Seems that Slack is no longer selling in China mainland, neither by SFDC nor by Alibaba Cloud. However if your are a Hong Kong entity, then you can purchase via Alibaba Cloud. Yes I am gonna try to change the country setting for our workspace too... Not sure if this would work.