Update 17 Dec 2018: I mentioned earlier that I will write a post to explain how you can modify your bot to handle this change. I noticed that Svetlana from the Dialogflow team has provided a comprehensive answer for how to deal with this change. I don't really have much to add to that.
Recently, it was discovered (unfortunately, there was no official announcement) that the template mode has been deprecated in Dialogflow.
People going back to older template mode training phrases were shown this message.
I intend to write a longer article about this in a later post, but for now, if you have been impacted by this change, please leave a comment below this article and explain how exactly you were using this feature.
In my second post, I will take a look at the questions/comments and try and provide suggestions as to how you can work around this.
Also note that there are probably going to be scenarios where there may be no way to replicate the behavior of the template mode using only the example mode, but from my experience it is doable for the MOST part.