Configuration changes are logged and can be reverted.
However, there are no instructions on how to see the changes log. I expect some of my nginx config to have been modified, but I do not see any changes. I'd like to inspect those in order to troubleshoot my issue.
It's a rare occasion when somebody asks about this! This functionality is usually only used directly by Certbot itself.
The backup files are stored in /var/lib/letsencrypt/backups/ and the changes are described in a file named CHANGES_SINCE.
Unfortunately the backups are only saved to disk for permanent changes, like installing a certificate. Temporary changes, like the ones done by the authenticator, are only kept in memory, and never saved to disk.
So unfortunately it will probably not help you in this instance.
If you really want to see the changes that Certbot makes, you can ask it to pause execution after making the nginx changes, with the --debug-challenges flag: