Driver source code/detailed hardware documentation for all the components. Signing keys to enable proper persistent installation of custom firmware. In a perfect world, source to whatever the final version of iOS to run on it is.
Basically, everything required for third parties to support the devices to the same level that Apple did before discontinuing them.
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(Getting downvoted for a strong right to repair stance? I wasn't really expecting that here, but OK.)
Right. It's totally feasible for a community-maintained firmware to keep these devices alive if there was no barrier to installing custom firmware, Apple ought to have a customary "EOL" update for these devices that allows them to be unlocked if the owner chooses.
Basically, everything required for third parties to support the devices to the same level that Apple did before discontinuing them.
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(Getting downvoted for a strong right to repair stance? I wasn't really expecting that here, but OK.)