One of the new features in Messages on iOS 11 is a dedicated app dock for your iMessage apps. You've undoubtedly seen it since updating earlier this week -- it's a row of app icons at the bottom of the screen each and every time you open a conversation.
I've seen a lot of complaints on Twitter, and have been asked privately a few times about hiding it. At first, I didn't think it was possible. Then I found the answer on Reddit.
Hide the app dock by placing your finger on the App Store icon next to the text field, and swipe down. Poof, it's gone.
There's a caveat, because of course there is. If you use an iMessage app in the future, the dock comes back. But you can repeat the quick gesture to get rid of it again.