Is there any technical reason why we now should put third-party modules in the [docroot]/modules folder?
Technical? Not really, it isn't faster or anything like that.

It's basically about Developer/User Experience. It's 2 clicks less to go there and find a module.

Also, there multiple issues to either drop multisite entirely (although most people disagree with that) or not exposing it by default and just have a settings/settings.php by default.


Move settings.php to /settings directory, fold sites.php into settings.php
Discuss whether to deprecate multisite for removal in 9.x