I’ve found that generally in Elixir and Phoenix we don’t pluralise (pluralisation does happen in other frameworks in other languages, like Rails, however it can get messy with edge cases, such as ‘octopus’!).
Naming conventions help a lot in frameworks, so for instance Phoenix will infer the name of certain modules from the name of others. For example it infers the name of the view, AppWeb.UserView, from the controller, AppWeb.UserController. Similarly, the view modules infer their template locations from the module name. So in the example in the Phoenix book, RumblWeb.UserView would look for templates in the rumbl_web/templates/user directory.
If you’re wondering why RumblWeb becomes rumble_web in the directory or why PageController in the module name becomes page_controller in the file name - I guess it’s for readability (and possibly inherited from the Ruby/Rails world - after I imagine quite a bit of discussion around it). There may be other benefits which I am not aware of though.
Once you get used to these conventions, it does make life easier as you don’t have specify them manually (although of course you can should you wish to break these conventions).
With regards to the pluralisation you found in Scenic, maybe @boydm could chime in and let us know his reasoning 
