Actually it is still reproducible. For example https://jenkins.ci.cloudbees.com/job/core/job/jenkins_ui-changes/ws/whatever.txt displays an error page in HTML but returns a 200 status code. Will see if I can fix it.
Searching the web for HUDSON-8950:
When a project has a workspace, but an attempt is made to retrieve a particular file which is not in it, DirectoryBrowserSupport returns a 404 as you would expect. When a project has no workspace, and an attempt is made to browse it, noWorkspace.jelly is served - with a 200 status code! This is unexpected and can cause problems for remote API clients such as NetBeans which use the ws/ URL namespace to retrieve workspace files: the file appears to exist but the "content" is actually an HTML error message. The attached patch seems to fix the problem without affecting the appearance in a web browser.
Apparently I submitted a patch that went into Hudson 2.1.1.