To add these features, marketers use a special code, called JSON-LD, in the header section of their emails. But with the recent update, the use of the <script> tag has become the standard. Currently, Gmail Annotations are still being tested, and only a select group of big brands can use them.
However, it's important to note that the use of annotations is currently restricted to an "allow list," with testing limited to mass emails. To apply to be included in this list, brands can reach out to Gmail's outreach team for evaluation.