From the commit history, 2015 (commit 5f7817c85d4b5f65626c8f49249a6c91292b8513).

I proposed the idea of improved compile-time checking and maintainability, as there wasn't originally much interest in a new hash function, but the maintainability improvements were something people could go for.

I hadn't spent as much time working on it as I am now, so it moved slowly. Other people also helped by converting parts of the code that they were working on (like parts of the refs subsystem).