The expression appears to be still in use, especially for the reasons stated below.
As per MacMillan Dictionary:
good boy/girl
used for praising a child or a pet when they have done something correctly
Have you done your homework? Good girl.
As per Merriam Webster
that's a good boy/girl/dog (etc.)
idiom
—used especially to praise a child or animal for obeying
Please pick up your toys. That's a good girl.
Sit. That's a good dog.
As per Cambridge Dictionary:
there's a good boy/girl/dog!
idiom (mainly UK)
used to show approval or encouragement:
Tie your shoelaces, there's a good girl!
Examples from the Corpus (Longman Dictionary)
• I am Pa's best boy.
• Randolph worked his hardest, pulling away, while Santa delivered all the presents to the good boys and girls.
• He's a good boy, and he's very strong.
• I tried to be a good girl and stay out of the way.
• He coughed, told Oliver to dry his eyes and be a good boy, and walked on with him in silence.
• Good boys, good boys, good boys.
• He had been a very good boy indeed.
Why would they not, unless they came from a Woke background?
– Robbie Goodwin Jan 27 '23 at 22:30