Is this desirable? This wipes other people's copies of their boosts, favourites, and bookmarked posts. If users want this behaviour, they can delete their own accounts.
how exactly do you propose someone looking to delete all of their old posts from a dead instance might obtain the exhaustive list of instances that have historically received their posts via a long indirect cascade of boosts to send these formal or informal deletion requests? an allowlist-federated instance obviously makes that far more tractable, and otherwise I could potentially see annoying paths to doing it anyway if i had been the admin and had every log ever sitting somewhere to trawl through, but in the case of a normal user whose instance is already gone and who doesn't have an ongoing relationship with the prior admin, this is hardly a realistic way to address that concern.