It is used almost entirely with gfm, the operation of gfm was in consultation. it was in this connection that the commonmark specification was formulated.
The pandoc repository[1] has nearly 45k stars and 4k forks. It's embedded in Jupyter notebooks, which are used by data scientists the world over. The python wrapper for pandoc has 16 million downloads[2].
But maybe that's insufficient. Hugo[2], one of the most widespread static site generators in existence, supports Pandoc markdown flavors as well as general GFM.[3]
They are not supported by GitHub’s GFM, and you are being incredibly bad faith about this fact.
If you are comfortable calling Markdown a futile exercise in avoiding proprietary control of a universal, simple document format for wide interoperability across natural languages, programming languages, and monopoly interests, then you and I are Microsoft vs Sun over Java in the 1990s.
The original Gruber-Shwartz format, which is in fact fundamentally a perl program attached to a brief expression of its intention, is not used by anyone except those who still use the perl program. Every other implementation avoids thousands of corner cases present in the perl, while of course producing its own.
The spec is the commonmark spec together with extensions of the type the commonmark spec was devised to make possible. People know better than to leave such a 'language' with a 'spec' that only really resides in a perl program, of all things. There are lists upon lists upon lists of corner cases the the perl does differently from every other program that, in the first years, reimplemented it. All of them show the perl does not meet the intention.
Swiss-army knife of markup format conversion
https://pandoc.org/
Installed (on request)
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/...
License: GPL-2.0-or-later
==> Installed Kegs and Versions
pandoc 3.9.0.2 (11 files, 274.7MB) [Linked]
==> Dependencies
Required (1): gmp
Recursive Runtime (1): all installed
==> Analytics
install: 31,898 (30 days), 119,598 (90 days), 369,388 (365 days)