So here is a more detailed answer that has been given to me on Stackoverflow, and that made me understand @LostKobrakai’s answer better .
By allowing subfolders for templates, you are basically saying to phoenix that each subfolder should be compiled into the corresponding top-level view.
This may not be a problem for a small project, where you don’t have many pages, but imagine you start having something more complex where you decide to nest views?
I don’t even know how Phoenix would react in this case, because basically that would mean your template will be compiled in two different views…
If it doesn’t work, you might spend time figuring out why, and if it does, you might at one point use the same method name in the top-level and sub-folder views, in which case it will no longer be clear which one you are referring to.
So I deduced that I can use subfolders safely as long as their master view is not nested in another view. My real concern with subfolders, it’s just to easily find small parts of a large template by organizing them by semantics. Sometimes I have a very big template and I prefer to divide it in many small templates. Of course they could just be placed in the same folder as their parent template, but I may end up with too many templates in the same folder. So I really prefer to create a subfolder for each big master template so I quickly I know where to search for its small parts.