Despite that petition being denied, Part 47 97.309[1] is pretty clear that as long as the mode's "technical characteristics have been documented publicly" it's fair game, subject to 97.307(f) [2].
This doesn't mean you can't prototype or experiment on the ham bands.
Provided you stay within the limits of your license you are welcome to experiment all you like.
Additionally, if you want to exceed the normal limits of your license then you can apply for an experimental license.
If I'm reading it right, the FCC denied the petition because you're already allowed to experiment on the ham bands. The whole point of the ham bands is experimentation!
That petition doesn't say what you think it does.
Many things have been prototyped in terms of communication modes. Just a few examples WSTJ, FT8, PSK31, MSK are all developed by hams for hams.
And there is along and continuing tradition of building your own radio.