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Zstandard was also mainly designed and produced by a single person, Yann Collet.

Yann also developed LZ4

Yes, but they also work for megacorp Facebook, and according to https://github.com/facebook/zstd/graphs/contributors, 300+ other contributors have made 4500+ commits to the zstd repo.

It's not quite as small-scale as 90's-style shareware was.

zstd has improved a lot since it was brought in under the facebook roof, but those are incremental changes, it already existed before then.

I recommend the CoRecursive episode about how LZ4 and zstd came to be. It started with making games for a HP calculator. https://corecursive.com/data-compression-yann-collet/