I recently updated to Nextcloud 20 and found an error in the log for several installed applications:
{“reqId”:“2gzVPhOnlozA3SoyVV3u”,“level”:0,“time”:“September 25, 2020 18:47:48”,“remoteAddr”:“xx.xxx.xxx.xxx”,“user”:“xxx”,“app”:“notes”,“method”:“GET”,“url”:"/ocs/v2.php/apps/user_status/api/v1/user_status",“message”:"/appinfo/app.php is deprecated, use \OCP\AppFramework\Bootstrap\IBootstrap on the application class instead.",“userAgent”:“Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36”,“version”:“20.0.0.8”}
This is a warning directed at the developer of the notes app. Unless you are willing to participate in the development, there is nothing you can do about it. But this is also merely a warning, it does not affect your Nextcloud instance in any way.
And as kesselb said, you should not have log level at 0, because you are getting a lot of information that does not concern you as an administrator.