alt Hacker NewsPeople interested in alternatives to Lean should also look at Metamath. It has nowhere near the adoption that Lean is getting, but is holding its own in [100] theorems results.
It has some advantages and compelling properties, not least of which is that it's very simple, so much so that there are many implementations of checkers; most other proof systems are ultimately defined by a single implementation. It's also astonishingly efficient — the entire database can be checked in less than a second. Set theory is also a familiar foundation for mathematicians, though the question of which is a better foundation compared with type theory is very controversial. Mario Carneiro pushed forward the development of Metamath in his thesis [0].
There are downsides also, including junk theorems, and automation is weaker. It's possible that types really help with the latter. Even so, I think it's worthy of study and understanding.