Ages ago I read on devRant about someone who set vim bindings for his entire Linux desktop.
Does anybody remember this rant or knows how to do it?

What WM/DE are you using? You pretty much only have to configure the one you're using to suit your needs.
If you also want the "Vim feelings" in other apps, I suggest using some that offers them by default (e.g. QuteBrowser).

It's going about a freshly installed Ubuntu 18.04, so Gnome Shell and all defaults.
I am not the one trying to achieve this but a friend of mine who wants to change from Windows to Ubuntu completely (he has quite some experience in Linux though) and misses his AutoKey configuration where he could use vim bindings literally everywhere, in all applications, including the browser and other desktop applications.
We have tried around with shortcut settings, but had some troubles with it (especially because Capslock and Escape could not be assigned).
Then I remembered the rant about something very similar long ago.