We still recommend OSX 10.10 for a month or two longer.
The Phase One firmware updater and USB3 for some Phase One digital backs are not yet compatible in 10.11. Both of these should be addressed soon.
In general, unless you just like being on the bleeding edge, waiting for a month or two after the release of a new Operating System is just common sense. You drop your chances of having annoyances/problems from "decent chance" to "very little chance".
That said if you do enjoy the bleeding edge, must update to 10.11 for some specific reason, or have already updated to 10.11, then yes Capture One 8.3.4 (the software itself) now supports 10.11 El Capitan.