While they were beloved characters on the show, there wasn't a specific storyline where they disappeared or anything dramatic happened to them. They were just part of the regular cast, and their appearances were part of the show's regular rotation.
However, here's some information about the characters and why they might seem "missing" from the later seasons:
* The Show's Format: Barney & Friends was known for changing its cast and storylines quite frequently. New characters were introduced and others would appear less often over time. This was done to keep the show fresh and interesting for viewers.
* Stella's Role: Stella was a main character in the early seasons of Barney & Friends, appearing in a variety of episodes. As the show went on, she appeared less frequently, possibly because the creators decided to focus on different storylines or simply to give other characters more spotlight.
* Mr. Boyd: Mr. Boyd was primarily a recurring character who was a bit of a mentor figure to Barney and the kids. His appearances were less frequent than the main characters like Barney, Baby Bop, and BJ.
It's likely that the characters you're thinking of continued to appear in some episodes of Barney & Friends, just not as often as they did in earlier seasons. If you'd like to see if they are in any particular episodes, you can search for them online or look for them in the show's credits!