In com_redirect in the Joomla admin, there are disabled redirect entries that shouldn't be there. I'm certain that no one put them there. The column heading is "expired url". How do these get there, and will it cause a performance issue that we have thousands of them?
Yes, Joomla records every URL request resulting in a 404 HTTP status code in the redirect component. The purpose is to provided an automated means for website administrators to manage redirects with the automatically logged 404 HTTP requests. More info on the component in the link below.