AFAIK, we didn't.
I just tried it and it appears with the free tier.
If it doesn't appear for you, and as Google Groups are reserved for general purpose discussions, I suggest you open an issue on issue-tracker sharing a screenshot showing that your integration button is missing (without sharing any sensitive identifiable information such as projectID, emails, ...). Make sure that the screenshot also shows that your agent is a free tier Dialogflow ES.
Additionally, let know in the issue if you tried from an incognito window. Sometimes extension conflicts can generate strange behaviors from the browser. I doubt it could delete a button but it would be good to confirm anyway, just in case.