You can serialize a long/ulong base typed enum with SaintsSerialized, which is not supported by Unity.

1. IMPORTANT: Set your MonoBehaviour/ScriptableObject to partial
2. Add [SaintsSerialized] to your enum field

// IMPORTANT: partial class

using SaintsField.Playa;

public partial class MyBehavior: MonoBehaviour
{
    [Flags]  // no need to use Serializable, because Unity just can not serialize it.
    public enum TestULongEnum: ulong
    {
        None = 0,
        First = 1,
        Second = 1 << 1,
        Third = 1 << 2,
        All = First | Second | Third,
    }

    public enum TestULongEnumNormal: ulong
    {
        None,
        First,
        Second,
        Third,
    }

    [FormerlySerializedAs("MyOldName")]  // FormerlySerializedAs works too
    [SaintsSerialized] public TestULongEnum ULongEnumPub;
    [SaintsSerialized] public TestULongEnumNormal ULongEnumNormalPub;
    // EnumToggleButtons is supported too
    [SaintsSerialized, EnumToggleButtons] public TestULongEnum ULongEnumPubBtns;

    [Serializable]  // use inside class/struct requires `partial` keyword too
    public partial class MyClass
    {
        [SaintsSerialized]  // This work inside a normal class/struct. Note the `partial`!
        public MyULongEmun myEnum;
    }

    // We don't need `SaintsSerialized` for this field
    public MyClass myClass;
}

This can work with EnumToggleButtons.