Everything is possible. Quantum immortality refers to subjective anticipation, a ranking of future observations according to their importance for future plans that one can prepare and carry out with an impact on environment (utility). Since an agent can't personally carry out plans in possible worlds where it's dead, these worlds have low priority in reflection on possible plans, thus most of anticipation is concentrated where the agent survives, however improbable that is.

On the other hand, one doesn't prepare plans for carrying out in the past, and so the heuristic of subjective anticipation doesn't apply. There is no salient decision-theoretic measure to be concentrated in the possible worlds where the agent operated in the past.