SHIELD knows about Phantom but they give him a low level threat classification because they think he can't leave his haunt.
This impression stems from a few curious Avengers coming to meet Danny and insinuating that he'd be a good addition to the team. Danny is thrilled at the idea but then thinks about how much Amity would suffer without him and has to reject their (unofficial) offer. He says
"That sounds amazing, but as much as I'd like to, I can't leave my haunt."
What he means is "can't leave my haunt unprotected" but his denial gets interpreted literally. As in Phantom physically cannot leave his "haunt". Maybe he even escorts them to the edge of town and is like "Well, this is as far as I can go, but it was nice to meet you all. Stay safe and keep protecting the rest of the world :)" that of course being as far as he can go and still get back in time for his next class.
“It was an important story-point to have those two different types of martial arts styles sung throughout the journey of Shang-Chi in this movie. One style is obviously from his mother, and then the other, more forceful traditional martial arts style is from his dad.” - Destin Daniel Cretton
Bucky: Okay Morgan, there are 206 bones in the body, but! I’m gonna teach you how to dislocate someone’s body in 230 ways—
Sam: Dude. She’s a kid.
Morgan:…
Bucky: Oh yeah! You’re right.
*turns around for a few seconds before turning back with puppets over his hands*
Bucky, in somewhat a Kermit the frog’s voice: Hi kids! We’re going to show you how to dislocate someone’s body! Let’s start with the Clavicle region. Do you know where that is?