Interesting. I don't think I've ever seen "appendixes". As far as I'm concerned :
appendix --> appendices
index --> indices
codex --> codices
When I proofread, I would correct appendixes.
Leave the 'xes' alone in all of them! :-D
I will certainly continue to convert the endings to "-ces", in the same way that I'll use "necropoleis" as the plural of "necropolis" (not "necropolises" or "necropoli"!).
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