This is just a normal drag and drop a!dropdownField with no modifications to it, and it's got the lightbulb icon next to it.
Yet the problem seems to be that the function isn't supported by Portals in this version, which I have checked and happens to be 24.3. It won't let me publish the portal with the dropdown there, however I've found a workaround in multi-dropdown, which portals are strangely fine with. Still curious to find the cause, as we may want portals with single dropdowns at a later date.
Appian bug?
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Yes, this is a known bug and will be fixed in the next major release. In the meantime, I believe it should still let you publish the page with the dropdown field despite the flagged warning.