How do I use Messages with Google Hangouts on MacOS Mojave? After upgrading to MacOS Mojave, Messages no longer allows me to sign in to Jabber / Google Talk.  Previously, if the Jabber account was offline, it offered a dropdown to sign back in.

Now, that dropdown appears to be gone.
You can't. Apple removed the option to log in to Jabber with 10.14, Messages is iMessage-only now.