Mathlib was fully ported to Lean 4 (https://github.com/leanprover-community/mathlib4 https://github.com/leanprover-community/mathlib4), and the Lean 3 version i

Mathlib was fully ported to Lean 4 (https://github.com/leanprover-community/mathlib4 https://github.com/leanprover-community/mathlib4), and the Lean 3 version is deprecated now. Edit: Adding a little bit more detail, it was ported one file at a time, using the automated translation from mathport (https://github.com/leanprover-community/mathport https://github.com/leanprover-community/mathport) as a starting point.