This was detected in a unit test in the Pants repo, but is a wider problem for all versions of Pants that support generating lockfiles using Poetry.

The proximal cause is this announcement from PyPI:
https://discuss.python.org/t/backwards-incompatible-change-to-pypi-json-api/17154

And the root cause is this Poetry code:
https://github.com/python-poetry/poetry/blob/bce13c14f73060b3abbb791dea585d8fde26eaef/poetry/repositories/pypi_repository.py#L272-L283

There was a Poetry fix released and backported to the 1.1. branch here:
python-poetry/poetry#5973

Users can fix with 2 steps:

1. Update Pants config

[poetry]
# N.B.: Works around issue described at https://github.com/pantsbuild/pants/issues/16111
# Undo once on a Pants with this version or greater as the default.
version = "poetry==1.1.14"

2. Clear Poetry caches with rm -rf ~/.cache/pypoetry on Linux and rm -rf ~/Library/Caches/pypoetry on Mac.

This issue tracks bumping Pants default to this fixed Poetry version.