currently minversion pyproject.toml[[tool.pytest.ini_options]minversion doesn't apply when running on pytest<6 because those versions don't parse the file.

I've found a work-around using a setup.cfg because pyproject.toml takes priority over setup.cfg:

# pyproject.toml
[tool.pytest.ini_options]
minversion = "6.0"

# setup.cfg
[tool:pytest]
minversion = 6

secondly it would be useful if pyproject.toml took priority over pytest.ini to allow pytest 4-6 to have forward compatibility using pytest.ini instead of setup.cfg which have meanings to other tools

This is not possible to do in a plugin, because minversion is useful assuming you do not have control of the packages installed yet.

see #7653 for https://pypi.org/project/pytest-forward-compatibility/