Here's a Lean 3 development of a bunch of topos theory https://github.com/b-mehta/topos/tree/master/src , but it's not in the maths library (and now needs to be updated to Lean 4, although the community have had great success with that kind of project; one million lines of mathlib was translated from Lean 3 to Lean 4 using a combination of automation and human work)