I do not believe this is against the rules.
Why?
1. It is not abusing a glitch
2. It is not abusing a bug
3. It is not a mod, hack, or modified/edited client.
4. It is, as some might say, a "Vanilla" Windows feature - Meaning just like how the sprint key in 1.7.2 is Vanilla, Sticky Keys is a "Vanilla" windows feature.
5. It is virtually impossible to tell if someone is using Sticky Keys or just has a rock on their keyboard. It would do the same thing, assuming the rock didn't fall off or something.
6. You can't change windows while doing it. With BSM you can (I believe) and that's what makes it particularly unfair. If you put a rock on your keyboard, you can't even go into your inventory, because it will un-sneak.
I could go on, but you get the point.