Yes, I looked into it, and basically everyone uses type: group_list.
However, my requirement is to select only one. If I use type: group_list, I have to add a custom validation to prevent multiple selections. While this works, it feels very awkward.
Why was type: group removed? Just like with type: category, there are scenarios where only a single selection is needed. It shouldn’t be a blanket rule.
Unless type: group_list can support the case of selecting just one.
I’m not sure. As the TODO notes, while working on the importer script, I noticed that the type group seems to be stored as a text, not an ID. I guess it was never implemented as a select. In the beginning of Discourse we mostly had text fields for site settings. This might still be a remnant of it.