It's web only. Most mobile devices support user-specified substitutions at the OS level or keyboard level, so people can easily set up a substition if they want. Not as easy on web, so the easter egg.
This post is about something new in Hangouts web chat. Just start a message with "/shruggie" and it'll turn into ¯\(ツ)/¯
If you'd like the same on mobile, the links I posted explain how you can set up text substitution to turn whatever phrase you'd like to turn into whatever you'd like, including turning "shruggie" into ¯\(ツ)/¯