I think you’re mistaking the plugins in config.php as system plugins.
Whether a plugin is listed under “System Plugins” is if it has the 4th field set to true in the init.php code of the plugin itself under the “function about()” array (first is the version, 2nd is description , 3rd is author, 4th is am i a system plugin).
The plugins in config.php are those that are Forced to be ON/enabled, users cannot disable them.
System plugins also cannot just be enabled or disabled by users.
This way, if you wanted to, you could force enable a User plugin for all users.
I think the general rule for what is listed as system and not would be if it is a back-end setting that cannot be selected per user, like authentication mechanism for example, would probably break if set to user but not enforced in config.php.
However normal user plugins have no issue with being turned on/off per person.
Note just so happens to be enabled by default i guess because it’s pretty unintrusive and adds some nice functionality that a user could just as well ignore.
Honestly I disable it myself, but I’m the only one that uses my TTRSS install.