Not the entirety of the future relationship, but the nature of the relationship on leaving is to be planned for during the negotiation stage:
the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union.
Naturally the nature of the future relationship will have great bearing on the negotiations surrounding an agreement on the arrangements for withdrawal. To take three possible situations: A country that wishes to leave the EU, but remain in the EEA and Schengen area will have a very different arrangements from one that has become hostile and wishes to side with a military enemy of the EU (imagine a country that wants to become part of a military bloc that seeks to invade EU states). Another situation is one in which a country was being forced to leave the EU: a constructive dismissal in which the EU has made it practically impossible for the country to remain.
Different situations calling for very different arrangements. The negotiators need to know something of how Britain will relate to the EU when drawing up the agreement.