I'm guessing I should be slightly embarrassed to have not realised that existed - was it there before they changed the top bar to black a few days ago? (I now see that there's a link to it from said bar...)
It kinda is. Across most Google products it is (gmail, music, calendar, etc). Most hotkey JS libraries I've seen also use ? as the default for seeing all of the bound hotkeys, so anyone who uses a library should have it by default too. It's always the first thing I try on a website.
I'm guessing I should be slightly embarrassed to have not realised that existed - was it there before they changed the top bar to black a few days ago? (I now see that there's a link to it from said bar...)