YouTube issued a massive security update in 2021 that made older unlisted videos (uploaded before 2017) "Private" unless the owner opted out. Many extensions may no longer work for these older videos. Alternative: Using the Unlisted Videos Website
When an extension claims to "show unlisted videos," it typically does one of the following: show unlisted videos youtube extension install
Though primarily a downloader, its "Media Discovery" popup will list every video on a page—including unlisted ones that are playing—allowing you to copy the secret URL. YouTube issued a massive security update in 2021