Yep! Headphones is good now. I would suggest that headphones are enabled by default on the stable build so that users with standard V4 kits will get expected behaviour. I would also suggest the option to disable be pushed down the admin menu. This does not seem like an option that is likely to be toggled often.
Note to users: Enabling headphones does increase CPU usage by a few percent. There may be use cases where this pushes you beyond the limit and causes xruns (clicks on all outputs). You may wish to disable headphones for such cases.
Note to technically minded users. The reason for the increase in CPU is that jack2 (the audio interface layer) binds to one physical audio interface and clocks at that interface’s samplerate. Any other audio interfaces will use their own hardware clocks which may drift hence there needs to be samplerate conversion (even at same samplerate) between jack2 and the extra audio interfaces. (There are ways to lock some (professional) audio interfaces, e.g. word clock but that does not exist for the RPi builtin audio interface used for headphones or most of the audio interfaces used by Zynthian for main output.)
There you go - more than you wanted to know but hopefully useful stuff.