register_sidebar_widget is deprecated

I was having a glitch this morning (title links broken) that somehow healed itself. Along the way, someone tried to help by running the debugger, which then displayed a longer version of the message in the topic title at the top of each site page. It also prevented my logging in to change anything, such as deactivating plugins, by displaying that message on an otherwise blank screen instead of a login.

I found the location of the debug setting and changed it to false, so I have functionality, but clearly something is still wrong that I should fix. I’ve searched these forums and seen suggestions, but I don’t know what file would contain the code to change. I’m not aware of even having such a widget, and the only plugin I have are Askimet, Jetpack, and Theme Switcher. I saw a thread saying the problem is in Askimet and not in the theme, resolving the issue by making the change to askimet.php. However, I located that file, and it does not contain a reference to the widget. So where else and what again do I change? Thanks.