zh-my, zh-sg, zh-mo, zh-tw, zh-cn, zh are no longer maintained as an interface language variant, thanks to the fallback scheme. currently maintained ones are: zh-hant zh-hans and zh-hk. It should be stressed although these are disabled, the existing have not been removed from the system, which cause maintenance overhead to downstream administrators. Suppose a system message have all the legacy variants set in mediawiki software, it means the interface administrator of a site need to create all the 8ish pages for a single message, which fallback to default mediawiki message instead of its fallback message on local instance.
So if MSG/zh-my and its identical MSG/zh-hans exist in mediawiki software, and the MSG/zh-hans is updated to use some templates maybe. MSG/zh-my needs to be created as well, otherwise it fallback to system message.
Until Translate wiki remove all the legacy messages, it would be best if we do not pull from there for these disabled variants.