Deletion of STS-135

Comment: Admittedly this is tough. STS-135 does exist. It is not a flown mission, nor a planned mission; it is however a planning construct -- an alternative being given consideration. Because the Space Shuttle program operates so much in the public view, there are multiple sources which, taken cumulatively, adequately attest to the existence of STS-135 in that sense.  The article needs to make all that clear in its lede, and above all avoid  claims
Stance: keep
Cited policy: Wikipedia:What Wikipedia is not (WP:CRYSTAL)