In the previous version of Google Analytics (Universal Analytics) annotations were included in your Google Analytics property. From your GA reports, you could annotate Google Analytics data with short notes on specific dates and share GA annotations with other team members who had access to the GA (UA) account.
This was an incredibly useful tool for businesses to see how activities were impacting their websites, such as new email campaigns, design changes, new content, promotions, new affiliate links, offline marketing strategies, real-world events, or anything else they wanted to document.