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[[Rezk completion]] seems like it should be a reflection since it comes with a canonical map forgetting the inherent equality by weakening it to isomorphism and a [[univalent category]] seems like it would be a local object wrt maps that just weaken the inherent equality. However, in that case it's kind of surprising this isn't mentioned on either nLab page. Is this an oversight or is it not a reflection after all?
Yes, univalent (-)categories are reflective in pre-(-)categories (aka Segal spaces in the -case), and the reflection operation is Rezk completion.