Anything that is story-locked, and isn't meant to be accessible via just throwing together a bunch of the right TFs.
For example, demon race should only be gained via taking Lyssieth's offer.
This is why they fixed the slime method to become a demon - demonic parts now revert to regular human parts on slime-to-flesh TF.
For a while, demonic horse parts still worked when returning to flesh, as an oversight. I haven't checked if they fixed that recently or not.
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