It's true that XDK is not the only one build system we can use, but it's - if I'm not mistaken - the only one officially supported at the moment?
No, we always have, and still do, support PhoneGap Build. I actually prefer it myself because it's simpler than the XDK - just upload a zip and download an .ipa/.apk back - but for some reason it doesn't seem to have gained the same popularity as the XDK (guess because the free version is limited to 1 app, but you can replace it with different ones!)
Canvas+ was never a real browser engine, and we dropped it as part of dropping support for the non-browser engines which were always incredibly painful to work with. Ludei have other platforms which are based on browser engines now.