Thanks to @lunchin pointing me that new Commerce version came out with improvements.

So the changes. First of all, custom promotion processor should inherit proper base class:

• for OrderPromotion use OrderPromotionProcessorBase,
• for EntryPromotion use EntryPromotionProcessorBase,
• for ShippingPromotion use ShippingPromotionProcessorBase.

Another change is that you are not able to new up new instances of RedemptionDescription, but instead, use base processor's method CreateRedemptionDescription.

Here is the same promotion I showed last week but updated to the latest version:

[ContentType(
    DisplayName = "Additional Discount promotion",
    GUID = "E6271950-DB98-4FE6-9626-CEFCBF46BE19")]
public class AdditionalDiscountPromoData : OrderPromotion
{
}

public class AdditionalDiscountPromoProcessor
    : OrderPromotionProcessorBase<AdditionalDiscountPromoData> // <-- Inherits from specific base
{
    protected override RewardDescription Evaluate(
        AdditionalDiscountPromoData promotionData,
        PromotionProcessorContext context)
    {
        var orderForm = context.OrderForm;
        var cart = context.OrderGroup as Cart;
        if (cart == null)
        {
            return NoReward(promotionData);
        }

        var additionalDiscountPercent = (decimal)(cart[Constants.AdditionalDiscountPercentMetaField] ?? 0.0m);
        if (additionalDiscountPercent == 0)
        {
            return NoReward(promotionData);
        }

        return RewardDescription.CreatePercentageReward(
            FulfillmentStatus.Fulfilled,
            new[] { CreateRedemptionDescription(orderForm) }, // <-- Changed RedemptionDescription initialization
            promotionData,
            additionalDiscountPercent,
            description: $"{additionalDiscountPercent} % discount applied to order as set by support staff");
    }

    private RewardDescription NoReward(PromotionData promotionData)
    {
        return new RewardDescription(
                FulfillmentStatus.NotFulfilled,
                Enumerable.Empty<RedemptionDescription>(),
                promotionData,
                unitDiscount: 0,
                unitPercentage: 0,
                rewardType: RewardType.None,
                description: "No discount applied");
    }

    protected override PromotionItems GetPromotionItems(AdditionalDiscountPromoData promotionData)
    {
        return new PromotionItems(
            promotionData,
            new CatalogItemSelection(null, CatalogItemSelectionType.All, true),
            new CatalogItemSelection(null, CatalogItemSelectionType.All, true));
    }
}

Summary

New API now ensures that promotions can be built only the right way - appropriate RewardDescription for each promotion type.

But I am not sure that the change is good from the API correctness standpoint. All base classes implement IPromotionProcessor interface and it's method Evaluate returns RewardDescription which requires RedemptionDescription in the constructor. But RedemptionDescription has an internal constructor and can't be created outside of EPiServer's base classes. Because of that, no other IPromotionProcessor interface implementations can be created. While it is unlikely that someone would like to create another base class for promotion processor, creating a stub implementation of it for unit tests is impossible now.