You're looking at StableBuild's Deleted PyPI mirror - where we index all deleted packages from the Python package registry.

itk-filtering
5.1.0.post1

This version was deleted from PyPI around 2024-04-18. Installing fails with:
$ pip3 install itk-filtering==5.1.0.post1 ERROR: Could not find a version that satisfies the requirement itk-filtering==5.1.0.post1