Somewhat strange and curious, but from yesterday to today Apple has placed on the App Store four packages of stickers intended new version of the app Posts (Messages) of iOS 10.
Just them: Smileys, Hearts, Hands and Classic Mac. The first three come from watchOS, while the last (with classic icons from the 1980s, designed by Susan Kare) is even embedded in the current and only beta version of iOS 10.
Version 1.0.3 (21.2 MB) Requires iOS 10.0 or superior
Version 1.0.3 (5.3 MB) Requires iOS 10.0 or superior
Version 1.0.3 (7 MB) Requires iOS 10.0 or superior
Although they are public on the App Store, it is obviously not possible to install them if you are not testing iOS 10:
Also a little confusing these packages of stickers be in the App Store itself, alongside normal apps / games. Let's see if, until the official launch of iOS 10, they are in a separate store specific to Messages.