What does it mean when a CCed 3rd party person was removed from the conversation?
The sender of that message removed the CCed person from the correspondence.
This can happen openly, for example when that someone was in charge of a specific phase or task that has been completed and is publicly removed from the conversation.
This could also happen behind the scenes in the form of a request by the removed person.
The implication of removing a person from the conversation can be viewed in light of that person’s role in the interaction:
If this person had a specific task or assignment that was completed, then this may be a good sign. However, if the task was not finished or there's a risk that this person may have had a bad opinion on the deal you're offering, then you should be more alert.
If the removed person didn't have a designated task or a functional role, then this might serve as a warning sign.