Source archives (e.g. in [1]) are available in tgz and tar.xz formats, but documentation archives (e.g. in [2]) are available only in tar.bz2 and zip formats.
I suggest that documentation archives be available also in tar.xz format.

[1] http://www.python.org/ftp/python/3.3.4/
[2] http://www.python.org/ftp/python/doc/3.3.4/

Do we need documentation archives in tar.xz format?

The format (xz vs bzip2) might not matter much, but I'd say consistency does. I'd make the formats identical just for the sake of standardization.

Where is this decided? In some (post)build script?

Those archives seem to go only up to 3.9. Is this still relevant?

Documentation downloads are still built and provided for each release (as linked to from https://www.python.org/doc/ and https://www.python.org/doc/versions/) as well as daily for the heads of each active bugfix and feature branch, in each case via the "Download these documents" link on the top page of the release/branch documentation set, for  example, https://docs.python.org/3/ -> https://docs.python.org/3/download.html. The downloadable doc formats are still .zip and .bz2 while cPython source archives are still .tgz and .xz formats. Julien Palard (@mdk) is the release team documentation expert.  Julien, what do you think?

I don’t think that consistency or standardization really applies here, it’s not like there are tools that want to process both source and doc archives and are broken by the difference in formats.

It could be useful to review the formats used and decide if they are still the best choices (maybe zip for universality, tar + modern and well-supported compression for size), but consistency for consistency’s sake is needless churn.

Oh to counter my own proposal: formats should not be removed without warnings, there are probably downstream tools that expect specific URL patterns and formats.  So adding something very modern to get the smallest size might be good, but not removing existing format without warning.

https://docs.python.org/3/download.html says a) tar.bz2 can be read by the tar program, which I presume is true also of tar.xz.  So there is no issue of *needing* two separate programs.  It also claims "The .tar.bz2 archives provide the best compression and fastest download times."  This only need be true, if it is, for the doc formats, as opposed to python code.  In the absence of at least 2 requests from *nix consumers of the archives, why even think of changing?

tar is modular and depends on optional external programs to handle compression (tar itself does archival only).

gzip is still common, and xz has largely replaced bz2 these days.
I would expect it to be installed on most systems.

(For HTTP compression there are new competitors, brotli and zstd, but I haven’t seen these used to compress files.)