GP mentioned this fork of 7-Zip with zstd support:

- https://github.com/mcmilk/7-Zip-zstd

The changes to the 7-Zip file format were discussed upstream, and upstream agreed to not tread on the magic values:

- https://sourceforge.net/p/sevenzip/discussion/45797/thread/a...

But ultimately the patches were not upstreamed. It's now onto its second developer:

- https://sourceforge.net/p/sevenzip/discussion/45797/thread/6...