I don't believe the Clip+ (or any other device) has ever had that functionality in Rockbox, however booting up the Clip+ with either Left ("rewind") or Home held down will boot the original firmware.
Also, if you have the latest bootloader, plugging the Clip+ into USB will boot the OF, however this will eventually not be the case, once USB transfers are supported in Rockbox, and so shouldn't be relied on indefinitely.