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if the debts are written off through a pre-pack sale - that normally leaves a large black hole in the pension fund.

There's now no defined benefit fund for Maplin employees, so there will be no fund to have a black hole. But even when there is a pension deficit (say Carillion), the costs of filling the hole don't fall to taxpayers, they come out of the Pension Protection Fund, which is paid for by levies on all the other private pension schemes. Meaning that the well run companies with solvent pension schemes have to pay for the incompetence of crooks like Carillion.

Taxpayers are of course on the hook for the perfect black hole of public pensions, which have in aggregate no significant fund, and thus (if accounted honestly) would be operating with a deficit equal to their future liabilities. If a private company did that its directors would go to jail.