There are preset module selections for the different workflows. So if you select a display-referred workflow, expect display-referred modules and vice-versa. I’d expect any exceptions to be there for a good reason and at a position in the pixelpipe where they won’t cause problems in normal use.
That already removes for me any need to visually mark the display-referred modules (and cluttering the interface).
And as @MStraeten pointed out, usually there’s no need to know if a module is scene- or display-referred.
You can of course make your own module layout. If someone does that, it’s up to them to ensure the combination of modules they use makes sense, and they know how and when to use them.
You can also modify the pipeline order. Again, if you do that, it’s up to you to be sure the order makes sense (hint: manual).
In both of these, I still see no need to clutter the interface for everyone else.
Note: Something similar (or identical?) was proposed already…