If you do this code:

// Optional OU references support

@include_once($CFG->dirroot- .- '/local/references/oucontent_assets.php');

you get an error:

819: File is being unconditionally included; use "require_once" instead

In this case I am specifically including the file conditionally - in other words, I am happy for the file not to exist, that is why I am doing include_once instead of require_once. If the code doesn't exist, I don't want an error.

I see what the test is for but I think there should be an exception if you use the @ symbol (which is necessary in this case, otherwise you get a warning when it doesn't exist) which indicates that you have considered that it might not exist. In other words if you do include_once you should still get this error but if you do @include_once this should be allowed.