Not always. Some caution is needed here. If the devices are associated with the plugin, then yes, the devices will go when the plugin is uninstalled. But it is possible to create devices that use the plugin code but are not associated formally with the plugin.
The best way to tell is to go to the plugin under Apps > My Apps and look at the list of devices. Make sure that list is complete – account for every one. If not, you’ll need to manually delete any device that uses the plugin code but isn’t listed before uninstalling the plugin. Otherwise, those devices will cause startup errors in your system, and also not show in the UI, so you’ll have a tougher time getting rid of them.
The association between a device and a plugin is established not just by the device type, but also by the
plugin attribute on the device. Any device can use the device type and implementation file of the plugin, but it is only associated with the plugin if the
plugin attribute has the ID number of the plugin in it. It is easy to create a device without this link: any time you use the “Create device” under Apps > Develop apps, you are creating an unassociated device.