On Thu, Jun 28, 2018, at 6:27 PM, Brett Cannon wrote:
The file was originally meant to target building wheels for libraries. It just happens that folks have pushed that out to include apps as well. So if the purpose of the file expands to include apps as well then that changes what the PEP should require to be in the file.

It even expanded to include tools not directly related to packaging at all - towncrier was given as an example, and that's a developer tool for producing release notes. I guess we implicitly invited this generalisation by calling it 'pyproject.toml' rather than anything explicitly tied to building or packaging.

In any case, I think it's too late to say such tools shouldn't use it, and as Nathaniel pointed out, it's pretty surprising that using a tool like towncrier would suddenly change how pip deals with your package. So I think the best option is to say that the presence or absence of pyproject.toml is not significant for build frontends - rather it's the presence of build-system information in that file that matters.