I have seen a number of (like, many many) videos where annotations were used for corrections on the content, so now all of that content is going to be wrong :(.

I realize that these things are sometimes annoying, and that mobile users couldn't see them... but they existed, and were part of the content--in many cases I have seen, because the video publisher didn't know how to do editing and so used the annotation feature to do it instead--and now that content is going to be damaged, forever.

FWIW, this was effectively the only way to fix content after it had been posted without changing the URL... something you would expect to be a critical feature of any publishing platform, and one that Google themselves has used at times and offered to close partners because clearly they need that feature themselves, but which YouTube doesn't really provide and apparently has now decided to provide even less :(.