This is Jenkins 1.399. If I go to "Manage Jenkins" --> "Manage Plugins", and then the "Installed" tab, at the bottom of the page is the text "Changes will take effect when you restart Jenkins", and a button with the text "Restart Once No Jobs Are Running". This is always displayed, even if I restart Jenkins. It shold only be displayed if there are pending changes (such as updating a plugin).
It's worth noting that in my Jenkins, I have the Subversion plugin pinned at an older release that the revlease bundled with Jenkins. So I don't know if this message erroneously appears for everyone, or it it's caused by my pinned plugin.