Is a Boot Camp partition in Parallels Desktop still natively bootable?
Yes. I do it all the time.

The only thing to remember is you have to properly Shut Down the Parallels version before trying to boot the Boot Camp version.

Full instructions - Parallels KB - Can I run my Boot Camp partition with Parallels Desktop?

One thing it notes is that it only works if your Boot Camp is on the same drive as your Mac partition. Mine isn't, it's on another drive entirely, though still internal. I can't remember whether I moved it before or after setting up Parallels; it was initially on the same drive when I first made the Boot Camp.