Logitech MX Keys: sleep mode keyboard shortcut
As Sleep exists as a menu command, you could make your own…

System Prefs > Keyboard > Shortcuts > App Shortcuts

Select All Applications, press the  +  button, type Sleep then pick a key command - I'd pick something hard to do by accident, like maybe  Cmd ⌘   Opt ⌥   Ctrl ⌃   Shift ⇧   S  



 I used D in the example image, because I already have S used for Shutdown