At age 11 it is normal to know some sexual terminology - because there will be lessons in school, but also you would expect children of that age to be asking questions out of curiosity and for them to be speaking about sex and relationships to some extent with their peers.
What you wouldn’t expect is a child to be using sexually explicit language towards another child - it’s hard to comment fully without knowing exactly what she said OP. However, age 11 you know right from wrong and I assume she knew what she was doing wasn’t right?
My child knows how babies are made but she does not know explicit sexual language other than the basics of penis, vulva, vagina, sex etc I’m guessing from the OP that the language used wouldn’t necessarily be expected for a child of her age? Knowing about sex in year 6 is appropriate - messaging people in a sexually explicit way is not. Knowing about sex in the reproductive sense in year 6 is appropriate - watching pornography on her phone is not, and neither is having an extensive knowledge about sexual acts etc.
again, more info is needed to fully comment on how worrying this is
Google the Brooke Traffic Light Tool for an idea of what is normal in terms of behaviours and what is not.
I know you are shocked but the best thing you can do is to help her to navigate having a mobile phone and being online, rather than take it away from her indefinitely. A year is a very long time not to have a phone especially if she is going in to secondary school. I think giving her a basic phone is a good idea as a starting point - that way when she out she’s can contact you and she can also text friends etc. Rather than punish her for what she has done though, use it as an opportunity to talk to her about what has happened. About why it wasn’t ok, about why the other child was upset etc. if some of the language used was worrying then you need to explore with her where she has heard the words / seen the acts. If she takes on board why everyone around her has been concerned by this then I think you need to work towards giving her opportunities to prove that she can be trusted.