The default depth for Xvfb was changed from 8 to 12 on 2005-03-23 by Roland Mainz and its manual page was changed accordingly. The code change was reverted/disabled on 2005-07-23 by Stuart R. Anderson but the manual page was not reverted.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
I've just pushed a fix to git. Thanks for your report!
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.