How to make flake8 fail if a plugin is not installed?
currently there is no such feature -- but in flake8 5.x (the next released version) there will be a (name pending) --require-plugins option
your best bet at the moment is to either (1) search pip freeze for flake8-docstrings (2) search flake8's --version output for flake8-docstrings
$ pip freeze | grep flake8-docstrings
flake8-docstrings==1.6.0
$ flake8 --version | grep flake8-docstrings
4.0.1 (flake8-docstrings: 1.6.0, pydocstyle: 6.1.1, mccabe: 0.6.1,
disclaimer: I'm the current flake8 maintainer