In Joomla there are different plugin types/groups - System, Content, etc.
Also events are separated.
onAfterRoute is described as System event.
Can I use
onAfterRoute from within a Content plugin?
All the Joomla events are categorized for different purpose. And each event trigger will be available only for those plugins which actually serve that purpose.
If there is an authentication type event then that event is available only for the plugin which belongs to authentication group.
Same goes with content related events like onContentPrepare.
But all the Joomla events are available in System plugins no matter event belongs to which category.
System events can only be used within system plugins. So you can not use the same within content plugin.