[kh99]

There is a userchangelog table, and assuming that the user was moved in some way that logs the change (i.e. using the user datamanager), then there will be an entry for that change. I think you could have a plugin that queries to find users that were moved to that group around 30 days ago, and decide if they should be deleted. I think it might be a little tricky because if the user was moved back and forth within the past 30 days, then you might find a record that says they were moved to the invalid group 30 days ago, but they might not be eligible for deletion.

I've been thinking about this because someone else asked about kicking members out of a "publicly joinable" group after a certain amount of time.