It was likely deleted because Stack Exchange is not interested in supporting the sectarianism of having two separate Islam sites. The proposal in itself is just a complaint that Shia and Sunni users are not treated fairly on that site. The Stack Exchange teams have addressed this issue there and have expressed their desire not to have separate sites for both of them. It's just not going to happen.
Stack Exchange is not a place for fighting over religion, and if the users of the Islam site really can't come to work together between both versions, then it's far more likely that Stack Exchange will just close down the site.
And if it is not possible to ask and answer questions on this topic in a polite, peaceful fashion, then it has no place on Stack Exchange.