True, but Pipfiles are basically whatever pipenv wants them to be. I'm not aware of anything but pipenv that uses them, although I'm sure there are more than zero such things. My point here being that I wouldn't take a lack of commits to the Pipfiles repo to mean that they're not still under development. More likely it's just being developed in the pipenv repo.
But in any case, I'd vastly prefer the standard pyproject.toml over some file specific to mostly one tool.
> This repository contains the design specification of the Pipfile format, as well as a proposed implementation of a parser for the specification which can be used by Pipenv and, in the future, any other consumer (e.g. pip)
It should probably be updated to reflect whatever the current status and goals are.
https://github.com/pypa/pipfile "This format is still under active design and development", last commit "2 years ago". I think this is dead.