Let's be honest, here: vocal jazz is a small subset of jazz. It operates by different rules, because as another poster implied, not all vocalists are the same. One vocalist simply might not be physically able to sing a certain tune in a certain key by no fault of their own. That's a completely different thing than, say, a sax player not knowing a standard in all 12 keys (and in fact, one of the lesser but still talked-about reasons for instrumentalists knowing standards in all 12 keys is so that when a vocalist calls out a tune in their best key, the instrumentalist can do it with no problem). If you're trying to limit this discussion to vocal jazz, that's fine, just say so so that we aren't talking past each other.
And as I said in another post, the whole "learn your standards in all 12 keys" is such a commonly-held value that at this point it's a trope. It's such a trope that I can't even think of a programming trope (to better fit this discussion to this website) that is on par with it. "Javascript has too many frameworks," "learn C (or Python) first," etc, I dunno. "All 12 keys" is such an extreme trope, especially in jazz education and history, that I struggle to see how you could call yourself a jazz musician and not run into it left and right.
> "good" is not a binary thing. I don't think I've ever met a pianist who was truly indifferent to what key to play in (Generally they prefer flat keys to sharp keys, and fewer flats to more).
Preference for a key != ability to play in a key (the former is historical/"best-practice"/etc, the latter is personal). I'm a guitarist/bassist/pianist, and yes I'm 100% indifferent to keys (though obviously some keys can be harder to sigh-read than others). That goal exists for a reason -- to achieve freedom over the things you have control of. I'm absolutely not alone in that. Literally every single peer of mine in school and since, regardless of instrument, held the goal of achieving total freedom in any key, in terms of sight-reading, improvising, comping (if applicable to their instrument), whatever.
Again, this difference in experience might be due to the fact that you're a vocalist -- you're a different breed, especially in education (for instance, another way in which vocalists tend to differ is their relatively extreme usage of solfege -- vocalists are heavy users of it, often to the point that some can only really "think" in it, whereas the majority of instrumentalists I know dislike it, even completely disregard it, and prefer to talk in chord tones/intervals. I'm sure you know this, I'm just pointing it out for anyone else who's reading this and is unfamiliar with the difference).
>> PS no one is gonna call out F#. They might call out Gb though. We always err to the side of flat keys, not sharps (horns like flat keys more).
> …which would tend to contradict your claim above that good jazz musicians can play equally well in all keys, wouldn't it?
Oh, come now. That's just being pedantic, and once again conflating reading ease with playing ease. My point was that in jazz one tends to prefer the enharmonic equivalent that favors flats over sharps for reading purposes. Obviously one should expect to play equally well in F# and Gb -- they're the same notes! Also, I was specifically talking about "all TWELVE keys," which in the context of a jazz discussion is limited to the common enharmonics of said 12 keys: going around the circle of 5ths (in the flats direction, as most jazzers spell it), it would be C, F, Bb, Eb, Ab, Db, Gb (maybe F#, but in my experience that's far less common, which is why I pointed it out), B, E, A, D, G. There's a reason I didn't include C# with Db -- it's hardly if ever used in jazz.