Where does YouTube's duration badge sit on a thumbnail?
Bottom right corner, inset about 4 pixels from the right edge and 4 pixels from the bottom of the rendered thumbnail. It is a black pill at 80 percent opacity with 12 pixel type inside it, so on a 360 pixel wide search result it measures roughly 38 by 20 pixels. Scale that onto the 1280 by 720 file you actually export and it becomes a block around 135 by 72 pixels. That is a lot bigger than most people picture, which is why the corner of a word or the edge of a chin so often ends up behind it.
What size is the YouTube thumbnail safe zone?
YouTube has never published one, so the honest answer is that the safe zone is whatever its interface does not reach. In practice that means keeping the bottom right corner and the bottom 4 percent of the height clear, and treating a 6 percent margin on all four sides as the working habit, because that also survives rounded corners and the crops different surfaces apply. If you also want the exact pixel dimensions for every YouTube surface, they are collected in the YouTube thumbnail size guide.
Does the red progress bar cover every YouTube thumbnail?
No, and that is exactly why it catches people out. The red watch progress bar only appears on videos the viewer has already started, so it never shows up while you are checking your own fresh upload. It is about 4 pixels tall on a desktop thumbnail and runs the full width of the bottom edge. Anything that runs into the bottom of your artwork, an underline, the base of a large word, the bottom of a subject cut out at the frame edge, gets a red line through it for every returning viewer.
Do I need a safe zone at the top of a YouTube thumbnail?
A small one, in the top right. Hover a video in the desktop feed and YouTube stacks the Watch later and Add to queue buttons over the top right corner, each about 28 pixels square with a dark rounded background. It is temporary rather than permanent, so it matters less than the badge, but a channel logo or the start of a face parked in that corner disappears under it every single time someone hovers. The mobile watched dim is the other one worth checking: the app knocks the whole thumbnail down in brightness for videos you have already seen, and a thumbnail that was already dark goes close to unreadable.
This page only looks at overlap. If you also want to see how your thumbnail holds up at 168 pixels, in greyscale and next to competing thumbnails, run it through the YouTube thumbnail tester.