My experience is the opposite of yours. With typescript I use an openapi spec to validate and generate types. For other entry points I use typebox to create validators and generate types. It requires a bit of work, but it adds a lot of capability even beyond basic deserialization.
With c# I've seen openapi interface generators that don't validate properly, only basic deserialization. I've seen dto's that are deserialized wrong due to lacking null checking attributes. I've seen the way put requests are misused due to how difficult it is to separate null and undefined in patch requests. I've seen dto's with all nullables due to the lack of union types. Maybe I've yet to see a good c# codebase, but I certainly prefer typescript over the above.
My preference is to have my types defined in code first, rather than in markup language files which generate the code. Having to rely on 3rd party code generation tools because your language lacks a feature is, as I see it, code smell. I'm not sure how to handle the scenario of undefined fields in a PATCH, but I also don't tend to write APIs that way
With c# I've seen openapi interface generators that don't validate properly, only basic deserialization. I've seen dto's that are deserialized wrong due to lacking null checking attributes. I've seen the way put requests are misused due to how difficult it is to separate null and undefined in patch requests. I've seen dto's with all nullables due to the lack of union types. Maybe I've yet to see a good c# codebase, but I certainly prefer typescript over the above.