You can load non-tool scripts in the Godot editor, if it's a direct load from a tool script. However, they may lose their static context in some cases if you do so.

When you run a project/scene and there are unsaved scene changes, the non-tool script will be reloaded, and static variables go poof!

So, always mark scripts as tool if they need to exist and persist in the editor. Even if it appears to work without.