This resolves No, retroactive to July 22, the day after Biden withdrew.

He was on the ballot for all primaries, and was voluntarily removed from the ballot for the general election.

To be clear, I'm not suggesting this as a question because I think it is especially likely. I know some questions related to relatively improbable events have been criticized here before. Nonetheless, I think one of the potentially valuable functions of Metaculus is that it can perform the probabilistic equivalent of a fact check (a "stat check?") on news like this.