Is the Keychain Access Menubar Item missing for anyone else?
The in-built Help app still claims setting the Keychain Access Menubar Item is possible through Keychain Access > Preferences > General. I no longer have a General pane in Keychain Access > Preferences and, thus, no way to choose to set the Keychain Access Menubar Item.
Do other people using 10.13 still have this option?
I'm glad you've found ways to lock your screens, but all of the initial responses were weirdly answering a question that wasn't asked. I rarely used the Keychain Access Menubar Item for the Lock Screen feature; my screen is set to lock after a short period of inactivity. I used the Keychain Access Menubar Item to access my keychain data and to quickly and easily create UserID/Passcodes outside of Safari and to create Secure Notes and to grab Certs.
Please, see my Crude Solution below.