How come they got rid of her character?

In real life, due to budget cuts perhaps.

In the fictional world of ABP, Stephanie was considerably older by season 4, plus Billie was there a lot too, so the show probably felt it was unnecessary to have her.

How come they got rid of her character?

She was not used nearly enough to show more sides of her character. The actress who played her excellent and was not able to do anything more than just smile at Archie for all of his foybles.
There was one episode, however, where she talked about her brother having been a victim of racism growing up. She was excellent!
It would have been even better if her and Murray hooked up and had them living in the house together ("for Stephie's sake" LOL). That would have sent Archie through the roof!

I've never seen an official reason, but as I recall Barbara Meek's departure was unplanned. Maybe she saw the writing on the wall?

Growing Up Is Hard To Do was kinda a mini finale of the Stephanie character. Not only was Stephanie on a lot less afterwards but her supporting cast disappeared. It was the final appearances of Rabbi Jacobs, Floyd Mills and BFF Amy Bloom. Father Barry Bloom would only appear in one more episode.

I'm not sure how they would have utilized the continued services of Mrs. Canby.

I think GameShowFan66 identified the reason Barbara was let go, budget. Her last appearance based on IMD was the two-parter which aired during the February sweeps of 82, Gloria Comes Home. In the second half of second 3 Barry Gordon was added to the cast. He made an early appearance in season 3 and then appears in five episodes during the second half of the season. He's a fulltime regular for the final season, so it became a numbers game.