What does it mean when a 3rd party person was removed from the conversation?
The sender of that message removed this 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 and hierarchy in the organization:
If this person had a specific task or assignment that was successfully 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.
Was this person in a senior position in the deal? If the communication continues with another senior person, it may indicate that the deal is still progressing. Nevertheless, if a senior person is abruptly removed from the conversation, that's probably not a good sign for the deal.
If that person is in a junior position, and the communication continues with a more senior person, it may indicate some progress in the deal.