AbstractPicoPlugin::$enabled now defaults to NULL what leaves the decision whether a plugin should be enabled or disabled by default up to Pico (precisely AbstractPicoPlugin::triggerEvent()). If all dependencies of a plugin are fulfilled, Pico enables the plugin by default. Otherwise the plugin is silently disabled (this was the behavior when AbstractPicoPlugin::$enabled was set to TRUE previously).
If a plugin should never be disabled *silently* (e.g. when dealing with security-relevant stuff like access control, or similar), set AbstractPicoPlugin::$enabled to TRUE. If Pico can't fulfill all the plugin's dependencies, it will throw an RuntimeException.
If a plugin rather does some "crazy stuff" a user should really be aware of before using it, you can set AbstractPicoPlugin::$enabled to FALSE. The user will then have to enable the plugin manually. However, if another plugin depends on this plugin, it might get enabled silently nevertheless.
No matter what, the user can always explicitly enable or disable a plugin in Pico's config.
Daniel Rudolf