Which I find to be a very practical solution for connecting to localhost over https, it frees you from having to install a self-signed certificates/CAs on your machine.
Not a great idea to publish private keys for valid certificates. Anyone could probably submit a certificate revocation request to the CA, as the key would be considered compromised.
https://letsencrypt.org/documents/LE-SA-v1.1.1-August-1-2016...