In the only docs I can find (which appear to actually be stale), a DropdownList should have a property
public virtual List<string> choices { get; set; }
In Unity 2021.1.23f1, this property is unavailable. When I used Rider to get to the source, it showed choices as internal instead of public, leaving the only way to set the choices at all being creating the DropdownList in C# and inserting it manually into the tree. There doesn’t appear to be any way to put it in the uxlm file and then set the choices (in my case, the list of choices is generated by querying the asset database to find prefabs containing a certain type, to get around the “We’re sorry, ObjectFields simply can’t see anything but the built in classes or ScriptableObject” issue.
Was this a conscious choice? It’s very frustrating as I’m designing this editor.