Maybe an expected behavior but an obstructive and not logical one.
If I delete a banner module in module manager the extension itself doesn't participate anyhow. At least a deletion should be possible if the module is trashed already.
I simply don't understand the sense of this blockage and why Joomla is forcing a single workflow while purging a system. What "catastrophe" could happen when I delete a module of an disabled extension compared with an enabled one?
A simple info like we see it already would be sufficient.
I would say this is expected behaviour.
Disabling an extension should disable all its use.