Check out the "Youtube Lite" plugin
This more advanced plugin lazy-loads YouTube videos. I'm still keeping this repo alive because it seems to be a great place for people learning how to build plugins, but head on over there if you want something to use on production
NodeBB YouTube Plugin
This NodeBB plugin allows users to embed YouTube videos inline in their posts. See discussion thread for this plugin on NodeBB Meta Discussion for more details.
npm install nodebb-plugin-youtube