folders require index.html file ?

I believe It's not a "must" but "strongly recommended".

But for example with index deactivated i don't think index.html is required, just a precaution measure for not displaying the files inside that directory

The JED no longer requires it.

But imho the link from there to Nic Dionysopoulos's (Akeeba) site has ALL the info anyone could EVER need on the subject :D (and some)

hmm... sorry but it seems links didnt work? so I try again

http :// community.joomla.org/blogs/leadership/1737-the-jed-indexhtml-requirement-dropped.html

http :// www.dionysopoulos.me/blog/86-the-files-of-wrath.html