I remember from the old Unity days, if I press and hold the Super key, icons in the sidebar would display this number corresponding to the position of the icon in the sidebar to quickly open the respective application by pressing Super + Number.
The bug is that the numbers appear on screen, but only after you’ve already pressed Super+Number. They should appear on screen before you choose which number key to press.
I’ve already spent on day on this. It needs some deep mutter changes (or reimplementing some mutter key listening in GNOME Shell as a distro patch). If we bring a shortcut helper functionality in 18.04 on long super press, we will need this. However, we need to discuss that heavily with upstream to not have a huge and hard to maintain code divergence.