We have a need to ideally organize a long list of dropdown items into optgroups. We like to stay "Craft-native" where we can, so we'd like to use the native Dropdown fieldtype to do this. Maybe some sort of flag in the value field?

I too would love to see this as a native option - see what I did there ;).
@CreateSean I鈥檇 count myself in the same group.
That was the optimal response.
Added for the next 3.2 Alpha release!

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Added for the next 3.2 Alpha release!