Suggesting to remove the psr-0 requirement, but leave the requirement to have a valid autoloadconfiguration. Afaik yii does not care how classes are autoloaded, so some extension authors may want to use classmaps instead or if the extension only consists out of a single file "file" (Should make no difference, as the classmap is generated at composer update/composer dumpautoload). Also with PSR-4 on the screen this may be to strongly worded.