Let's say my multi-class Warlock 3 / Wizard 17 and his party have just killed his patron, fulfilling a life long dream to be free of an ancestral Pact with the evil fiend. He has zero interest in becoming 'sworn and beholden' to a new entity.
Now, according to this somewhat related question "What happens if the entity a warlock has a pact with is killed?", the accepted answer is basically "No patron, no power". That makes sense from the stand point of losing the ability to cast Pact Magic, as well as access to the various class abilities that were gained from the fiend patron.
However, there are still 3 character levels that it's unclear what to do with. These levels were gained through adventuring, so what happens to them? Does the experience gained just evaporate? Does it transfer to the Wizard levels?
Obviously, the easy way out is "It's up to the DM". What I'm actually looking for is if there has ever been a precedent set for what happens when a character basically loses access to a class. For instance, 5e Oathbreaker Paladin shows what happens to a paladin that breaks their oath.
I'm interested in finding any precedent set throughout the history of D&D that will help a DM decide how to handle this.
What is the precedent for what happens to a warlock's character levels if their patron dies?