Greetings,
FYI/ I've upgraded from Mavericks to El Cap and discovered an issue that was vital to me. For some unknown reason El Cap moves these two plug-ins from the plug-in folder to the sub-disabled plug-in folder, resulting in Safari alerting me that I needed to install a Quicktime plugin.
The warning provided a link to Apple so I went to the page, but there was no plugins. I was at a loss and needed access to my surveillance systems.
An internal search led me to the <Internet Plug-Ins> folder, inside the folder was another folder <Disabled plug-ins> and there sat two that appeared to be QTime related. Needing to view my cameras I risked relocating them to the active folder and presto, back in business again.
nsIQTScriptablePlugin.xpt
QuickTime Plugin.plugin
I don't know why this happened, so subsequently I searched for more info only to discover that others have had the same experience and solution.
That made me a little better, at least confident that my Mac wouldn't over react from the switch.
https://discussions.apple.com/thread/7263683Happy Holidays!
If I recall, Apple and everyone else, for that matter, is moving away from plugins. See:
Chrome has QT disabled by default as well, apparently.
I don't create QT content for websites any more - don't think most people do. Gone the way of the dodo, along with Flash.