Lists of abstract classes do not work.

[pudy248]

Notably, abstract classes on their own, and lists of interfaces both work, but lists of abstract classes in particular are ineffective. Arrays also don't work. Sample code to demonstrate the issue:

public class ReferenceTestClass: MonoBehaviour
{
    [SerializeReference]
    [SerializeReferenceButton]
    public Test1 singleClassTest; //Works fine
    [SerializeReference]
    [SerializeReferenceButton]
    public List<Test1> listTest = new List<Test1>(); //Does not show up in the editor at all
    [SerializeReference]
    [SerializeReferenceButton]
    public List<Test1> listTest; //Not instantiating the list has no effect
}

public abstract class Test1
{
    public int field1;
}

public class Test2: Test1
{
    public int field2;
}

[pudy248]

Turns out I'm actually retarded and adding a constructor and the [System.Serializable] attribute to each class makes things work fine.