What drush version I can use that doesn't require composer?
I believe any version on this page that is released as a phar file:
https://github.com/drush-ops/drush/releases

Super old versions of Drush (unsupported) were not released as single phar files. The current version of drush (version 9) seems to be geared to released/maintained via Composer only; and no longer Pecl or other distribution channels.