TL;DR: Watch one of the first 10 videos in your “Watch Later” list and see if it magically reappears.

Doing a web search for this problem reveals it has been an issue for some time, but possibly for varying reasons in the past. The above is working for me on Web + native clients as of 6/2021.

YouTube has a neat feature, ‘Watch Later’, which does exactly as it says on the tin. The Watch Later list has a very handy context menu option to ‘Remove Watched’, which will appropriately enough prune the list of already-watched videos. Or at least it’s supposed to. This option, which appears on the…. ugh… “three vertical dots menu” (is that really what you call these things?) on the current Android mobile client, appears to randomly appear and disappear from the native mobile client and Web versions of YouTube on a whim.

After more fruitless searching and plinking around than I’d like to admit, it appears this is not telegraphing the future removal of this option, nor part of a grand conspiracy to wean users off of it for more ‘engagement’ by manually deleting every watched video off the queue, instead it’s a dastardly combination of programmer cleverness and laziness. Cleverness in the sense of programmatically adding and 1984’ing* removing the menu item itself depending on an algorithmic assessment of whether it would currently be useful (vs. just leaving the menu item alone and having it do nothing if there are no watched videos to remove), and laziness in the sense of this assessment only checking if there is a watched video in the first ‘n’ (dozen or few) entries. Don’t ask me the exact value of ‘n’ or what factors it may vary on, but it does at least appear to vary between Web and native clients, so you may run into cases where the option appears on the Web version of YouTube but not the mobile app, and on another day disappears from both. Watching something in the topmost (oldest added) 10 or so videos seems to pretty reliably fix it though. (Just don’t ask me what this extreme menu-decluttering shenanigans is meant to accomplish, other than having you Google in circles at len…oh.)

* This menu option does not exist. This menu option has never existed.