Impa has the advantage of only one version of her being thrown into a Spirit event, and more than one design as battle capable(Oracle, OOT, SS) overall.
So her having support despite lacking a consistent design can be easier since at least there's multiple ways to make her work. It's pretty hard to completely stop her from being a plausible character. Not that it means people will think she's likely even then considering how the Zelda choices are set up, but in particular, disconfirming her entirely never felt as easy as some characters.
Even then, only two of her designs are generally considered plausible after HW due to popular opinion. OOT is clearly one of the most important games to Nintendo, so that helps. There's also opinions she could work as an Echo(I feel her bodyshape deviates too much regardless of the design, but eh), or even a Sheik replacement, etc.
I don't expect her support to die any time soon, and that's a good thing. She's a really cool character that has a lot of merits and options for a fun and unique moveset. And being it's confirmed she time travels, that can also mean it's easier to justify basing her abilities off of more than one game too, not too much unlike how Link was designed in Smash 64(or how Zelda is nowadays, since she doesn't represent only one design overall. Albeit, Link was more lucky in that regard that his design was pretty consistent all the way till TP changed more than just his bodyshape, and Zelda actually is a magical person to begin with. That, and they needed a quick move to replace Transform and it happened Spirit Tracks was relevant at the time, so the Phantom Knight, a thing specific to a version of Zelda, worked well).