hi,
I have the Menu Bar icon on my version
As you can see to the right, behind the edge of the chat area I have a Buddy List.
You need to be signed into a Buddy List (Only Jabber now) to then have the lower portion on the Messages Menu > Preferences > General Pane show up to be able to toggle the icon On and Off.
I initially tried to add Jabber from the item in the Messages Menu but this messes things up.
I did it there as in Sierra you had to Add the Google talk account in Messages before then ticking the Mail box in System Preferences > Internet Accounts.
I had to remove Google from the Internet Accounts to eventually remove it from Messages - along with deleting com.apple.iChat.Jabber.plist
Then I added Google back (There is no Box for Messages to tick).
Instead Add A New Messages Account under Other account (scroll the right hand side down in Internet Accounts)
This added the GoogleTalk account properly with Buddy List and Access to Messaging, showing as an Account in Messages as well as Internet Accounts.
(I used Automatically find Server and Port).
9:00 pm Tuesday; December 12, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)