The real answer to this is: it's up to HR and your manager. I shit you not, HR and your hierarchy are free to apply or not apply the law at their whimsy. A cute white muslim girl? Oh sure, your hijab doesn't bother us. A fat old black woman with a WWJD wristband? Kthxbye. There's no appeal process or anything for people denied employment, a promotion or a transfer over bill 21 issues.
Proponents of the bill claim it'd never happen or insist that you find documented cases of work interviews denied over such issues, which is simply impossible because the employer isn't required to document them. The fact of the matter is that it gives government employers free reign to discriminate and refuse employment or bar from transfers and promotions for existing employees.