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This is an incredible video ! I have know a practical understanding of the liskov substitution thanks to you ! TBH I am amazed by the development of type systems that brought algebraic data types and funcitonal design pattern like monads. With that, the code is fully syntactically documented and don't need manual check for liskov substitution since it's automatically detected by the compiler. I also agree that properties must be specified by types (since types are proof of the existence of certain attributes in type theory). And finally yes, I prefer abstract class rather than interfaces. I generally use traits in rust that combine a bit of both

Abstract base classes allow me to read code. Do you love stepping into empty virtual interfaces when stepping through code or debugging? Then use interfaces as much as possible. I think your rules here are great. Although I would just generalize it and say keep to the same coding style. Don't sometimes return null, Exception and empty list. Be consistent as much as possible.

wouldnt the last example @1:58 just be an example of bad practice? you define that you are going to return string, but null is not even a string type.does the static code checker really allow this?

I mostly use CPP where I find it very hard to use interfaces/abstract classes, because it is more work. I defined some templates which help me a bit, but it is still so much faster e.g. in java to add interfaces and abstract classes. Sometimes I use interfaces as well as abstract classes. First I create the interface, and then the abstract class. E.g. in a MVP or MVC design pattern.