I just changed a post from (I believe) the standard Pending Review status to an Extended Status that essentially assigns the post to someone for review. I think I unchecked the Pending Review checkbox too – then clicked Save. This triggered a Publish action which then emailed notifications of a new post to subscribers.

Can you explain the expected workflow, current button naming, etc, so that we know exactly what WordPress might do with posts as Extended Post Status is used?

On an existing post, we must click “Save as pending”. Do not click the “Save” button.
This is just the “Publish” button renamed. I’ve seen notes on this but the change is not valid. We expect Save to save it in its current state, not to go further than that an publish, essentially with the Published status even though we are using a non-Published extended status.
Only click Save when you want to Publish – perhaps while maintaining a non-default extended status, which can be useful.

There is still the Pending Review checkbox. We can leave that checked until a post is ready for publication, but I’m not sure yet when that is auto-set or not in the New > Draft > Update workflow.

For now I have disabled notifications and will experiment.