I agree. Now that TOML has reached 1.0.0, I think that updating PEP 518 (which is still the reference definition, linked from ) to require that version of TOML is the right thing to do.
Thanks for raising this, @hukkinj1 
On a procedural note, this is technically a backward-incompatible change to the definition. And unfortunately, the reference documentation for pyproject.toml is still PEP 518 (linked from the PyPA specifications page). We had a somewhat inconclusive discussion on process that ended up here - but that doesn’t really allow for modifying a PEP after it’s finalised.
I think therefore that this change would need a PEP, which requires pyproject.toml to conform to the TOML 1.0.0 spec, while at the same time moving the canonical spec to packaging.python.org so we can be more flexible in future.
But having said that, I’d also be happy to support a proposal that we find a better workflow for changes like this (which are so small in scope that a PEP seems over the top, but which have non-trivial interoperability implications). I don’t have the time to champion such a proposal myself, though…