Huh, and here I thought Google was one of the few manufacturers left that simply support it on their hardware. So it depends on some cloud service being alive.

Do you know if it's the same for Fairphone or Shiftphone? Or is there another manufacturer that doesn't require this?

I've recently bought a new phone so it's not relevant for me anymore but when I next go looking, it can factor into it. As it was, I had Google marked in my spreadsheet as the most accessible unlock method together with brands like Oneplus and Fairphone

It's probably worth pointing out that the online process is one time and it installs a token that permanently lets the setting be toggled offline afterwards. This persists across factory resets and flashing any OS.

I wrote more details about it works under the hood here: https://news.ycombinator.com/item?id=35856171

Epic level comment! Very interesting to read about in this technical detail, thanks for taking the time to write it up.