YouTube will delete existing video annotations on January 15, 2019
You'd think Google would prioritize shipping the ability to replace the underlying video file for a video since it's impossible to do so: https://support.google.com/youtube/answer/55770?co=GENIE.Pla...But no. You can trim your video. You can delete it entirely, and suffer the consequences. But you can't simply fix and replace a video that may have had erroneous content and videos which had been "patched" by annotations in the past will now appear without context (aside from descriptions perhaps).That's some impressively lousy product stewardship, and I say this as someone who mostly disliked annotations as most people 
here seem to.