Yes, it's that "send-what-i-just-typed-to-the-sender-of-the-message-I-just-got" button.
I am starting to wonder, more and more, how the seemingly change-because-we-can icons come from. Is there some kind of contest for "who-can-change-the-smallest-thing-and-completely-baffle-the-user"? Or maybe a "who-can-hide-the-most-controls-in-the-strangest-places"? Is "tech support" now getting paid by the number of calls they receive?
I don't know why they had to change that - like Gary, I looked at the screen scratching my head the first time I attempted to send a message in iOS 10. However, the icon does have the advantage of being the same in all languages, so maybe that was a consideration. Who knows?