Get WP CLI to hide debug warnings and notices in JSON output, same setting as website

A quick manual solution is to direct all error output to a log file somewhere or even to /dev/null.

With your command this would look like this:

wp plugin list --fields=name,status,update,version,update_version,title --format=json 2> ./cli-command.err.log

If you totally don’t care about the errors, warnings and notices, you could send it to /dev/null like this:

wp plugin list --fields=name,status,update,version,update_version,title --format=json 2> /dev/null

Having said this, there is some chance that the plugin that you are still getting warnings from is actually using something different than the errors stream. My suggested solution will not help you, if ajax-event-calendar is using echo instead of error_log, but nothing will be better of a solution then than patching the plugin and notifying its author.