On Mac OS, I notice that there is a folder for plug-ins inside Adobe Lightroom.app. If I add a plug-in to this directory, will I lose it when I update to a newer version of Lightroom in the future?
Plugins are retained when you upgrade. However, there is no guarantee that they will work properly with the new version of Lightroom. If the plugin is still maintained, there will probably be an update if this is necessary.