The Memory Alpha article on Shelby quotes Ronald D. Moore discussing the mention of "Captain Shelby" in DS9 as saying there was some licensing issue involving the possibility of mentioning/seeing the character again. What is the issue? What is wrong with mentioning this character, or seeing her in a follow-up STAR TREK adventure?
The "Captain Shelby"/licensing issue was to do with Peter David's Star Trek: New Frontier
novel series. Basically, Pocket Books were assured by The Powers That Be that Shelby wasn't going to be seen or mentioned again in TV/film Trek, so they were free to do whatever they wanted with the character in the concurrent book series (which features her heavily, and I recommend if you're a fan of the character)
After TPTB "forgot" about their agreement and Captain Shelby was mentioned in DS9, one of the books had to include a bit where it was explained that the newly-promoted Captain Shelby was another
Starfleet officer named Shelby, not "our" one (who at the time was first officer of the Excalibur)