What drush version I can use that doesn't require composer? I try to use drush on the server that is behind corporate firewall, and downloads from many sites are blocked. In particular, when I run "composer diag", it hangs for the long time on the line "Checking http connectivity to packagist: http://packagist.org/" and then prints "The "http://packagist.org/packages.json" file could not be downloaded: failed to open stream". I saw similar blockage when trying to use wget with a number of URLs, including ftp-origin.drupal.org/files/projects/.

So how far back should I go to get the version of drush that doesn't use composer and will not try to update itself on the first run? I understand that some functionality will be missing, but at least basic functions like "drush cc", "drush en", "drush dis", etc. I will be able to use.
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.