Nice catch!

This file is very rarely read now, so this isn't the best way to adjust the defaults (it will apply to only some installs). Instead:

• Adjust the global default in src/applications/config/option/PhabricatorDeveloperConfigOptions.php.
• Adjust the default from conf/default.conf.php, or remove it entirely (if removed, it will fall back to the global default).
• Override it in conf/development.conf.php.

I think this never got set to true by default because it used to rely on building complicated externals (notably, jsxmin). However, we now have reasonable builtin/fallback minifiers and setting it to true is sensible.