After upgrading a site from Magento 2.1.x to 2.2.3, log output is no longer being written to
var/log/debug.log. There was nothing in the 2.2.0 change log about this behavior.
Magento 2.2.0 added a "Log to File" option in STORES > Configuration > ADVANCED > Developer:
By default, this option is disabled. If you enable this option, content will be written to
var/log/debug.log again. If you are in the backend of a site in production mode, you won't see this configuration field. You'll need to manually change this setting in the database using a query like this:
INSERT INTO `core_config_data` (`scope`, `scope_id`, `path`, `value`) VALUES ('default', 0, 'dev/debug/debug_logging', '1');
Note #1: caution should be taken before enabling logging on a site running in production to avoid private data being written to this log file.
Note #2: Logging behavior for shipping/payment methods is changing in 2.2.4, per this note in the change log: Magento now provides dedicated payment and shipping debug log files to store information specific to those functional areas.