Annotations are great in Google Analytics and other tools. You can make a note in the reporting tools on a specific day or data point what happened that you think caused the blip in the data. So if you do a huge redesign one day, annotating that in your reporting tools can be useful when you look back at the data years later.
For now, Google Search Console does not allow you to add your own annotations. You can click anywhere on any of the reports to add reminders, notes or annotations about specific days or data in that report.
To be clear, Google does add their own annotations to the report to communicate reporting glitches or data changes:
But you cannot add these annotations yourself.
John Mueller of Google said on Twitter that Daniel Wasiberg, he colleague, has been trying to convince the Search Console folks that this is a good thing too” to add but he said “but it sounds like it’s a hard sell.”
So who knows if we will get it.
There is this tool from SEOtesting.com that can do the annotations for you I believe. But it is not built in by Google by default.
Forum discussion at Twitter.