The chime is in the instrument cluster itself.
The key in ignition switch is internal to the ignition switch and not a switch mounted on the ignition key cylinder like prior model year Super Duty trucks. From reading the WSM, cutting the key in ignition switch wires coming out of the ignition switch should disable the brake shift interlock system which means you will not be able to shift the transmission out of park and possibly affect other vehicle functions. The sense circuit is hardwired to the SMJ (Smart Junction Box) and the SMJ communicates key in ignition status to the Instrument Cluster by network communications. You could TRY cutting the Blue and Gray wire coming out of the ignition switch and see what happens but I am pretty sure doing so is going to cause problems.
I do not see any customer configurable options for turning it off in the Owner's Guide. There is one last possibility and that is to see if the Key In Ignition chime if configurable using a scan tool - the Ford IDS. I could only answer that if I have a 2008-2010 truck handy and my IDS connected to it.