I am not aware of a clear definition for the term zombie channels. At least I don't recall the term being used in the lightning rfc. Also I didn't hear lightning devs in conversations to talk about them. Thus I think the semantics are more along your suggestion that they refer to channels which are for some reason obsolete / inactive / unusable. Disclaimet: just because I never saw the term doesn't mean it has no definition

Most of the articles that I have read refer to channels/nodes that haven't sent channel_update or node_announcement message for a long time (often weeks) as zombies. They say, a node who wants to route a lightning payment cannot rely on a path containing these nodes (hence zombies).

But that is a fuzzy notion since nodes will not send these update messages if the user hasn't updated any information about the node/channel. That would be unsolicited and spam.

I think that is the very reason this term does not appear in any formal documentations/specs like BOLTs and as a result lack a formal definition.