protect_critical_users vs user_protect

In D6, this module is still valid if you just want to fix the problem in Drupal core. There are modules that offer more features, and so you can choose and opt for the option that better suits your needs.

This module will die with D6, because the real problem in core was fixed a while back: #46149: Prevent account cancellation for uid 1 (it took 3 years)