Where do I add an old address while writing a page?

There is an Old addresses panel beside the editor on every page and post. It lists the addresses that already lead there, with how many visitors each has brought, and takes one more. Administrators see it, and editors or shop managers once Who can manage redirects lets them in.

Renaming is handled for you, so this is for an address the page had before you installed the plugin — and the moment you remember one is while you are looking at the page, not three menus away.

If the address cannot be added (it is part of WordPress itself, for instance), the editor tells you why after Update: in the block editor as a notice at the top, in the classic editor at the top of the screen that comes back. An address that is added is confirmed too: in the block editor at the bottom of the screen, with the box emptied, and in the classic editor in the panel itself.