Reasoning
Competitive programming platforms exist because it's comfortable to write computations and the validity and speed of a computation can easily with high confidence be verified with another computation, that iterates over most relevant inputs and checks the corresponding output.
This mostly didn't apply for the verification of mathematical proofs. Until recently, with the creation of Lean the mentioned difference is becoming smaller. If Lean improves and becomes more widespread, a platform hosting theorem proving competitions could be created.
Resolution criteria
The platform must be used and follow a similar format of 'Codeforces' in particular:
Contests of length between 30 minutes to 1 day can be organized
Each contest can consist of multiple problems, and solving earlier and more rewards the participant
Participants submit a .lean file or some other format which contains the fully written proof, which can be verified by the host automatically using other computation
There have been at least 10 public contests
On average, in each contest there were more than 50 participants