HttpClient supports http2, the latest protocol features, has been rewritten in managed code with sockets, and includes advanced handling to balance connection lifetimes with DNS updates. It's about as modern as it gets and we run 10 billion requests per day through this code without issue.

How does a lack of support for an RFC (which is still being revised) that outlines where a cookie should not be accepted in edge cases mean the HTTP library is not modern? Where are you running into this issue in a real app?