We have built an archetype-based ECS, mostly building on the Arche ECS in Go lang. There are still a couple of features on the road map, but the central functionality is there. In benchmarks, Larecs beats Arche in almost all test cases :-).
I would be super happy if others were interested in joining, and I'd greatly appreciate feedback and ideas how to bring SIMD and other Mojo perks to the world of ECS.