I would guess this change builds on the existing json endpoints for package metadata but that the Ruby DSL is remaining intact.

I think how to marry the Ruby formulas and a Rust frontend is something the Homebrew devs can figure out and I'm interested to see where it goes, but I don't really care whether Ruby "goes away" from Homebrew in the end or not. It's a lovely language, so if they can keep it for their DSL but improve client performance I think that's great.