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?
UPDATE:
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.
Crude Solution
You must disable SIP for this to work.
I went into a Time Machine backup and found the Keychain menu: Applications > Utilities > Keychain Access {Show Package Contents} > Contents > Resources > Keychain.menu or /Applications/Utilities/Keychain Access.app/Contents/Resources/Keychain.menu.
I copied this menu into the same place in the 10.13 Keychain Access app and double clicked the copy to make it appear in the Menubar.
And it works, without issue. For now.
UPDATE 2:
The modification persisted through the first High Sierra Supplemental Update. Hopefully, the next point update will include restoration of an intentional Keychain Access Menubar Item Please, to allow easy access Keychain Accesssee my Crude Solution below.