Sure do, mentioned under Rule of Three (pg. 86) and Unity of Rings (pg. 105).
You got it, under Torholt (pg. 100, brief mention on the right column). This ones in a bit of a grey area as I could see it being parsed as to indicate particular arcane's that won't enter but its strange to call out any race at all rather than just "customers" (or whatever) that won't enter Sigil. (Likewise the idea there's a suitable enough number that avoid it as to justify mention I imagine would be pointing to something deeper,.....or is simply wrong).Page citation on that?