Pardon me? I'm pretty sure that there are (occasional) in-video ads for channels not under YPP and explicitly-demonetized YPP videos even before 2020. (and I'm pretty sure that they were in no way claimed by others at all)
No idea if the creators opt in to this “feature” (I bet they don’t; it’s probably either all or nothing with ads), but it definitely exists.
You used to be able to opt-in to mid-roll ads. A few months ago, YouTube switched it to an opt-out system, helpfully enabling them for all then-existing videos that hadn't opted in:
> Starting in late July, videos that are longer than eight minutes will be eligible for mid-roll ads. As part of this change, mid-roll ads will be turned on for all eligible existing videos and future video uploads, including those videos where you may have previously opted out of mid-roll ads. Videos that already have mid-roll ads turned on will not be impacted.
(As I understand it, there is no way to disable the mid-roll ad on all videos, so you'd have to manually flip it for every single video. How kind of YouTube.)
I do not have definite proof for this, as I said on the top comment it is rather occasional, but I'm pretty sure that this happened to me on these videos (will update as I remember them):
- Tom Scott (which is definitely in YPP in 2018) has a video titled Stories I Can't Tell [1]. At the time I've watched it for the first time, it has a in-video ad at the tail of the video even though he explicitly said that the video is not monetised (and therefore it might be Google getting the money). Notably, he prepares his videos days or weeks in advance so I'm confident he indeed turn off monetisation on this.
- From time-to-time, I have accessed channels for work purposes. One of them is Fisher Scientific [2], which does have ads occasionally despite the fact that it shouldn't have (and obviously they wouldn't have bothered to submit YPP).
- Similarly, YouTube channels using Google Workspace accounts do serve an ad from time-to-time. Again, it is occasional, but from what my child has experienced, it does happen even with educational accounts (they have since moved to using Google Drive, although it is cumbersome since there are no features such as playlists).
I pulled up a private browser session to make sure I was not logged in, and the video was served ad free.