Usage of GNU's tar format

Hi,
In several different places Duplicity claims to use the GNU tar format:
"Standard file format: Athough archive data will be encrypted, inside it is in standard GNU-tar format archives." https://duplicity.us/features.html#xpointer(//*[@class="feature-list"]/descendant::li[4])

"The files used by duplicity to store backup data are tarfiles in GNU tar format."
https://duplicity.us/devel/duplicity.1.html#operation-and-data-formats

I was interested in seeing what I could do to change this for my purposes, but I can't tell where in the Duplicity source tree the choice of GNU tar is imposed and I'm beginning to doubt the accuracy. At https://gitlab.com/duplicity/duplicity/-/blob/dev/duplicity/dup_tarfile.py you can see that except for a shim for the sake of caching, Duplicity seems to use the standard 'tarfile' module that's part of Python. If Duplicity was relying on this module using GNU tar as the default format, it appears this changed in Python 3.8:

"Changed in version 3.8: The default format for new archives was changed to PAX_FORMAT from GNU_FORMAT."
https://docs.python.org/3/library/tarfile.html#tarfile.DEFAULT_FORMAT

Could this mean that Duplicity is actually using the standard pax format now? That's what I would really like to make happen. It's standardized in POSIX/Single UNIX Specification and is very extensible.

At https://duplicity.us/new_format.html#xpointer(%2f%2f%2a%5b%40name%3d%22nottar%22%5d%2ffollowing-sibling%3a%3aul%5b1%5d%20%2f%2fli%5b3%5d)
there is  a list of deficiencies in the tar format that are potentially mitigated by pax. The pax format is deliberately extensible and robust for advanced use cases, so it supports arbitrarily-long pathnames, extended attributes, embedding application-specific metadata, and more. I don't know if Python's tarfile module supports these advanced features well or at all, but if it so happens that Duplicity is using pax now with newer versions of Python, I'd like to see that change stick.

Please let me know where a place to start would be if I wanted to use ustar or pax formats instead of GNU tar, if that's not already happening.

Thanks

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