The current default.pem file in Arcanist is from Wed Jan 20 04:12:04 2016 and at this point is out of date
This revision replaces that .pem file with the one from here https://curl.se/ca/cacert.pem which is the Mozilla CA Certificate store from https://curl.se/docs/caextract.html
Specifically, this addresses any issues with the LetsEncrypt root certificate expiration of 9/30/21