Yup, we ran into that a while back. We were told there were plans to re-add it, but we haven’t followed up in a while.
We’re running 14.1X53-D45.3 and it’s not there.
In the current recommended release of code, half duplex on the EX4300 is supported and works.
I posted an update in the original article back in October. The ELS changes confused the issue with regard to the configuration.