Does OSX Mavericks prevent third party software from having a "Save as" option?
The answer to this question is complicated.  Way back in Lion, Apple began to offer optional API's to software developers that would make it easier for applications to manipulate different versions of saved documents.  The most noticable repurcussion of these updated APIs was the replacement of the Save As.. menu item by Duplicate and Rename... menu items.  For those interested in the details, the best guide is the relevant section of John Siracusa's masterful Lion review.  However, use of these new APIs was optional, and the old functionality is still supported in Lion, Mountain Lion, and Mavericks.  

Most non-Apple software still uses the old Save As menu for now, but in Mavericks some of the Apple applications have migrated to the new Duplicate menu.  As far as I know, Apple has not announced any plans to make the Duplicate model mandatory in the future, so this inconsistency is likely to stick around for a while.

Bottom line: In Mavericks, most applications still have the Save As... menu item.

Photoshop in Mavericks, using the familiar Save As... menu item:



Textedit in Mavericks, using the new Duplicate menu item: