The question is how are people obtaining h5py. Most people either obtain h5py from pip or conda.
PyPI (via pip)
https://pypi.org/project/hdf5plugin/
Conda-Forge (conda)
https://anaconda.org/conda-forge/hdf5plugin
The source repository for those packages is maintained by the synchrotron community via the silx project.
Additionally, zstandard is an open source project maintained by Meta (fma Facebook):
http://facebook.github.io/zstd/
These third party filters are generally open, free, and registered:
https://portal.hdfgroup.org/display/support/Registered+Filter+Plugins
The HDF Group maintains a repository that collects the source code of these filters:
There is an ongoing conversation along these lines in another thread: