As a salty wizard player, I don't think that "wizards have historically never gotten a useful robe so they can't have one now" is really a great long term strategy for that slot. In DnD, there are plenty of magic robes for wizards to use, and they historically enhance arcane abilities over providing defense. This robe is a perfect fit for that thematically.
As far as existing UR armor options for Wiz:
- Robe of the Archmage is outright useless, shouldn't be considered as a precedent.
- Moonhide is part of the starhide transmute chain, and really should mostly be considered in that context. This focus robe would be better, but there are literally two options.
For what it's worth, the starhide++ transmutes are probably better/more slot efficient still for wizards. The dex it provides to hit your AC 15 target opens up enough other slots. I'll leave it up to someone else to do the math though if they want, but anecdotally, I'm using something like 4 slots to hit my dex needs that the robe would handle.
If starhide++ is entirely outclassed, then it's probably worth asking whether or not it should be +4 in that slot period? AC isn't a real tradeoff to make 80%+ of the time, especially for about +4 focus at NM+ difficulty. You'd have to be trading off
much more AC than those slots are actually providing to even make that worth considering.