Google has announced one more new Gmail Smart Compose capability after this week's 15th anniversary update. Smart Compose will now begin suggesting subject lines based on the content of an email.

Posts from this topic will be added to your daily email digest and your homepage feed. If you are tired of Google trying to write your emails or documents for you, here’s how to turn this feature off.

I don't use the web interface, but I just logged in and it is definitely in the home tab for me (first thing on the left). if you don't see it there, on the home tab all the way to the right there ...