XKB's LatchLockState is missing the modLatches field

Per XKB protocol spec the LatchLockState request has the modLatches field. The field in xkb.xml is commented out, with a note saying it's a "workaround to prevent an API break":

<request name="LatchLockState" opcode="5">
	<field name="deviceSpec" type="DeviceSpec" />
	<field name="affectModLocks" type="CARD8" mask="ModMask" />
	<field name="modLocks" type="CARD8" mask="ModMask" />
	<field name="lockGroup" type="BOOL" />
	<field name="groupLock" type="CARD8" enum="Group" />
	<field name="affectModLatches" type="CARD8" mask="ModMask" />
	<pad bytes="1" /> <!-- This pad is a workaround to prevent an API break,
	                       which the following field (correct fix) would cause.
	<field name="modLatches" type="CARD8" mask="ModMask" />
	-->
	<pad bytes="1" />
	<field name="latchGroup" type="BOOL" />
	<field name="groupLatch" type="CARD16" />
</request>

What could that API break be, if it's a part of the protocol? Isn't it impairing the functionality of XCB?