How to disable Cmd-Option-number shortcuts to navigate to websites? On Safari, if I open a page with a URL and navigate to another page, Cmd-Option-8 navigates back and Cmd-Option-9 navigates forward, which is also the behavior in Finder.

But with a brand new tab and when the page does not have a previous or next page, Cmd-Option-8 opens Twitter and Cmd-Option-9 opens Linkedin.

How can I disable the shortcuts to these pre-defined websites?
By default (on my Mojave installation, at least), Cmd+Option+# triggers Safari to load the #th Favorite Bookmark. E.g. if you View > Show Favorites Bar, you will see that Twitter and LinkedIn are the 8th and 9th bookmarks. I would show the favorites bar and remove what you do not use.