new OS Sierra removed well-used icon?
I'm not in front of a Mac at the moment, but is the icon you want available when you customise the toolbar?

To check:


Within Mail go to View > Customize Toolbar (or you can right-click on the Mail toolbar and select Customize Toolbar)
The Customize Toolbar window will appear
Now you can check every button available for the one you're looking for (i.e. the Thumbs down icon - see note below)


NOTE: - The Thumbs up icon will only appear when you've got an email selected that Mail has marked as junk mail. Otherwise it will appear as a Thumbs down icon by default. In other words, what you're really looking for is a Thumbs down icon. Add this to your toolbar and you should be fine.

[EDIT]

Now that I'm in front of a Mac running macOS Sierra, I've determined that the icon you're looking for has been replaced with a new icon (see below):



Once again, this icon will change depending on the current status of the email you have highlighted. If Mail thinks it's already Junk, then the arrow points up instead so that you're removing it out of the trash. Otherwise the arrow points down by default so that clicking on it will trash it.