Many plugins set the minimum version to 5+ simply to avoid offering support for lower versions even if plugins work. Some functions were backported from WordPress 5+ to ClassicPress. I use Plugin Doctor - WPSeek.com to check plugins/themes for the minimum version based on the core functions used. When v2.0 drops, more plugins will work.
The Debug plugin requires a minimum version of 3.4, but based on the core functions, it can be used with 3.0.0. So should work with CP without issues.