Sinbad was a new Muslim superhero that Bill Loebs introduced into the Superman titles in 1990 during a fill-in arc on the Superman titles. The late, great Curt Swan came back to the Superman titles (that he drew for over 20 years in the 1950s through the 1980s before the books rebooted in 1986) and drew the storyline, which was about a young telekinetic hero from Qurac (DC’s stand-in for Middle Eastern countries like Iran or Iraq). Sinbad (which, okay, was probably a bit of a sketchy name, even if Loebs obviously meant well) even got his own entry in the 1990 DC’s Who’s Who…First of all, I don't think it was actually stated in the original story by Loebs that the character was a Muslim, though it was noted in the Who's Who entry he was Arabic. However, the profile does state that when he got hold of a belt that could provide him with some of the telekinetic powers he got, Lex Luthor waged an anti-Arab hatemongering campaign to try and retrieve it, dispatching paramilitary crooks to vandalize houses and stores in Little Qurac, including those belonging to Davood Nassur's folks. It even included a bizarre bunch of crooks in costumes who all called themselves "Sinbad", and Nassur made an effort to stop the imposters. The profile also notes that his family were victims of the secret police of the Shah in the country, another suggestion they weren't whitewashing Islam per se at the time (thankfully, the industry wasn't that far gone back then), but depending on your viewpoint, the notion Luthor would wage such a campaign as he did is still disturbing and questionable from an artistic perspective. Bill Loebs and Curt Swan were both pretty talented creators in their time, but this sounds like a potentially black mark on their records.
As for the Superman issue 712, where the reintroduction of Nassur was to have taken place, it was replaced by the story written by Busiek, and Roberson had the following statement to offer:
As much as I look forward to seeing an unpublished Kurt Busiek Superman story, it’s a shame that DC didn’t determine that the story we prepared for Superman 712 didn’t work in the Grounded storyline in time for us to do a different story. As it happened, the Sharif story was included in the outline for the remaining issues of Grounded that I submitted in November. The outline was approved, and in February the issue synopsis that I provided was used to draft the solicitation text, to work up character designs for Sharif (the grown up version of Sinbad from the early 90s), and for cover art to be pencilled, inked, and colored. The script for the issue was accepted in April, and was drawn, inked, and lettered. Unfortunately, when the issue was ready to be sent to the printer in the third week of May I was informed that the decision had been made not to print it.”I wouldn't be shocked if the story had been very pro-Islam as could be expected, and Roberson's already had his share of moments where he made leftist comments that were truly awful. That DC opted to drop his Superman story was certainly odd, considering it was surely never the first story they'd ever published with a whitewashed view of Islam (at the time, they'd published the joint project with the Kuwaiti propagandist behind The 99 comic) and look what happened with Green Lantern soon after. At this point, I'm sure the irony isn't lost on anyone that Geoff Johns, by sharp contrast, was granted full ability and free rein to introduce his own Islamic propaganda - much worse than anything Sinbad under Roberson might've stood for - in the form of the Simon Baz character in Green Lantern, and something tells me that if Johns had written the story credited to Roberson, it would've been greenlighted instantly without question. I honestly don't get why Johns, much like Brian Bendis, gets all these privileges, or why DC's willing to take "risks" with his ideas they won't with writers of ostensibly lesser caliber. But that's a telling problem about the Big Two nowadays: they have a lot of favoratism for overrated writers who get high profile jobs all because they've worked in some allegedly fancy job or written a supposedly classic product that wouldn't work with a larger audience.