Yep, back when DE wasn't sure if they wanted Warframes to have masculine and feminine models. But then they realized it'd be a lot of work (new models, getting cosmetics to work right, possible skeleton / animation changes, etcetera) for little reward, and decided the closest they'd ever do in the future is frames with similar powers and aesthetics.
"Old models / art" is also why Ember is often seen as futa / shemale by /d/, since she was originally slated to be a he. To a lesser degree, the same goes for Ash being a cuntboy (originally a female). Nothing official for any of the above three, just something people saw and ran with.>>6197793
Above extrapolation has lead to a variety of theories on how you change Warframes, too. Some people see it basically like a Venom symbiote, so no user-side changes. Some see it like a mould the wearer is shaped to fit in. Some see Tenno as energy beings (very few now that Conclave confirmed fleshy Tenno). Some that each Warframe is its own Tenno / being. And so on.
Only Mirage, Valkyr, and Limbo have any real backstory that I'm aware of, respectively: Lost in war against Sentients, fucked up by Alad V post-Cryo, and fucked up a Void ritual making their self go boom. And that's just the original, if you go by the theory that each is a watered down copy just like how a 40K Space Marine is a watered down Primarch.