Actually there is already kata for implementing Serializable mixin:
https://www.codewars.com/kata/learning-typescript-mixins

The intend for this kata was that programmer should know how to implement extend/intersect functions if needed. As you might know, extend function is already specified in the official TypeScript documentation. So I decided to create kata for similar intersect function. Just because it is easier to remember something when you implement it yourself (and this intersect function is almost the same as extend).

Good point! Thanks, I've made changes to this kata based on your suggestions!

Thanks! Great idea! :)
I've made changes to this kata based on your suggestions. Now it is entirely dedicated to abstract classes.

You're right! This kata was made only for the sake of the exercise. Just to make sure that programmer knows what "type assertions"/"double type assertions" mean...
For union types there is another one exercise here https://www.codewars.com/kata/learning-typescript-advanced-types-union-types
Subject "Union types" is a little bit more complicated than "Type assertions", so it's introduced later in this collection:
https://www.codewars.com/collections/learning-typescript