Sounds like virtualization. If that's your argument, you can argue that a CPU can be in userspace, I mean yes, but no
I was thinking more along the lines of network namespaces -- although actually, I should have been thinking of TUN interfaces. Any packets sent into one of those are delivered to the attached program, which can do what it likes with them. Those are usually used by e.g. OpenVPN for L3 things, but nothing stops a program from handling the L4 headers.