The Impress Edit menu has an "External Links..." item. It apparently used to be called just "Links..." - and the documentation still refers to it that way. It should be updated.
Added the Help page URL: file:///opt/libreofficedev25.2/help/en-US/text/simpress/main_edit.html?DbPAR=IMPRESS
Whoops, used my local URL; the web URL is: https://help.libreoffice.org/latest/en-US/text/simpress/main_edit.html?&DbPAR=IMPRESS
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/4f957ab38158f000cba832c0c9183b0a8544e16c tdf#163886 Fix "Edit > External links" for DRAW and IMPRESS