Basically, have you beaten the 0.5% odds, whether it was through FGs or Nutaku gold, and pulled a rainbow outside of guaranteed deals or the 20% rainbow tickets?
I'm tempted to say the first slice gacha shouldn't count either, but go ahead and specify those if you did.
In my case, I've pulled three rainbows legit. I didn't get shit during slice gacha though.
Alpinia from the 100 NG roll during Kerria's original spotlight.
Christmas Rose from a 10+1 pull during her spotlight. https://www.youtube.com/watch?v=bWHEm2IP01k
Dendrobium dupe from a 10+1 pull during Viola's spotlight. https://www.youtube.com/watch?v=AsFil6V7HOs