FYI: Matt Mackall is now Olivia Mackall [1], so that can make searching for things harder. Looks like they work at Valve, now? Agreed that they were a really stabilizing and healthy personality in the project, and made a lot of good early decisions, which Mercurial has continually benefited from. Eg: the commitment to backwards-compatibility [2].
[1] https://repo.mercurial-scm.org/hg-stable/rev/d4ba4d51f85f
Olivia uses she/her, and I believe she is now retired :)
I owe her and the Mercurial community a great debt. The community taught me how to think like an engineer building infrastructure and the importance of backwards compatibility, something I've tried to carry forward in tools like cargo-nextest.