If you’re looking at the master branch it was removed as I’m looking to discontinue it in the next release. The next release is planned to renew when 1/3 of the valid time remains or less than 7 days of valid time remains, whichever occurs first. The intent is to be more accommodating to the ACME service(s) of choice.
The current release still has it though:
# auto order certs with less than this number of days remaining of validity
'valid_remaining_days_threshold': 20