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

itk-numerics
5.3rc4.post4

This version was deleted from PyPI around 2024-04-18. Installing fails with:
$ pip3 install itk-numerics==5.3rc4.post4 ERROR: Could not find a version that satisfies the requirement itk-numerics==5.3rc4.post4

Install through StableBuild

You can install this package from StableBuild's PyPI snapshots from before the version was deleted:
pip3 install \ -i https://YOUR_API_KEY.pypimirror.stablebuild.com/2024-04-17/ \ itk-numerics==5.3rc4.post4
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Or, download files manually

And install manually via pip: