Bird maidens would just be cluttering up space that they could use for angels, the more popular and safer choice.
Angels? On an Egyptian-themed plane? Nooot convinced...
It's not that they are afraid to leave out angels if they don't fit, e.g. on Theros, Tarkir, Kamigawa... They could do all sorts of flying things instead that fit the look and feel better. Archons could easily be redesigned to look Egyptian, or even Lammasu. And Bird Maidens look that much like Ma'at that I'm not even sure it's entirely coincidental. Or they could go crazy and have soldiers in magical flying boats or chariots.
Honestly, Apophis/Apep is almost more of an Eldrazi than a demon. Apep isn't just evil, it's the Outer Chaos against which Set (normally a pretty bad dude) will gladly defend Ra, a monster that exists beyond and outside the ordinary theogeny, the eater of souls and destroyer of immortality that even the dead must be protected against, a thing so terrible that, fearing even depicting the entity would give it power, is only ever drawn with at least one other god there to subdue it. Power-wise I feel it would be more appropriate to depict Apophis as a God than as a non-god creature, even though that title is still somewhat inaccurate and somewhat insufficient. (God Horror? Assuming it's germane to the plane and not a representation of Emrakul or some supertitan that Emrakul, Kozilek, and Ulamog were just the brood lineage of. Maybe a being with god-like rules and no creature types at all.)
It partly depends on how the set/setting would handle gods in general, and on how much focus they'd want to give to ersatz-Apophis. If God is a creature type like on Theros, and they have the perfect slot in the pantheon for an Apophis-like deity, then that might be a thing. Or they could make him a Demon God (or Snake Demon God?). Dunno, the image of Apophis I have in my head is just more 'demon' than 'god'. And while he's threatening and powerful, he also gets killed by a god from time to time. That said, he has a thing for resurrecting himself and coming back, maybe that could be a thing for the card designers to play around with. A big evil snake of chaos and darkness should definitely be a thing on an Egyptian plane, I think the question of creature type would be secondary.