Update, having now deregistered post-jailbreak:
Everything in your documents folder is deleted. (To be fair, you get a warning about this when you opt to deregister.)
This means that not only do any sideloaded books get deleted, but so does the KUAL "book". So as dsip said, you have to do the ;log mrpi step again. After that KUAL should be available again with all the previously installed extensions. So far, KOReader seems to be working normally.
The hotfix "book" also disappears and I'm not sure whether or not that means you have to reapply the hotfix.
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