I tried \newcommand but clearly that's been disabled. Is there some way to quickly write Einstein notation?

 

[math]\eta_{\rho\sigma} = \Lambda^u{}_{\rho} \Lambda^v{}_{\sigma} \eta_{uv}[/math]

Where's the delete option?

You don't need to bother with the enclosing \[ and \]. As far as I'm aware, there's no quick way, but if you'd like me to define a new command, I can put that in and then add it to the documentation.