The races in the appendix of the PSHB are reinterpretations of 3.5’s races, however, keep-eyed observes note that the Xeph was omitted (so was the thri-kreen).
Unlike the maenad, I didn’t come up with a way to spin the Xeph as a psychic race that wasn’t just “it’s a human… with powers.” Maenad I was able to overhaul into a more distinct, even alien, race, and dromite I felt was well adapted to be more than just a reskinned halfling (honestly, I’m not hating on the Expanded Psionic Handbook writers, it’s just that the races felt too human to me). Elan was acceptable as it was easily a human subrace/variant, and Half-giant was fairly distinct (honestly, if WotC would have updated Dark Sun, I’d have cut the lovable brutes).
Thri-kreen I never considered because I figured there were dozens of homebrewed versions out there and there was nothing very psionic about them as a core race (if there is desire, I’ll incorporate them in a nonpsychic race expansion later).
This left the Xeph. In order to represent the Xeph, I felt I would need to write a completely new concept of them that skewed far from the 3.5 version that I felt would leave fans of the Xeph unhappy. I considered making it a gith subrace as it felt it could thematically fit, but dismissed it as potentially too controversial because since the era of Gygax there was Githyanki in the Astral Plane and Githzerai in Limbo and no other gith people. With the existence of the Elan, it made little sense to adapt Xeph as a human subrace, so this left it with no satisfying avenue.
If the Xeph is a race you want, what are your boundaries in editing it so it feels like a distinct race?