Put any video URL into the text bar (make sure it has ?v= in it). It should give you 9 thumbnails.
Thumbnails 1-3 are in 640x480 format, JPG. Thumbnails 4-6 are in 1280x720 format, JPG. Thumbnails 7-9 are in 480x360 format, WEBP.
It works on all public and unlisted videos, but also on rejected videos.
The best way to find rejected video URLs is to scan a playlist with MW Metadata Bulk and looking for ones that say "Deleted video". Additionally you can press Show Unavailable to get video metadata from Filmot.
Back to the YouTube Thumbnail Downloader