Apple II computers don't come with DOS in ROM, but rather need to boot from a floppy disk to make the disk drives usable. The IIc was the first Apple II that came with a built-in disk drive, so booting it was a must (it also omitted the cassette ports earlier Apple IIs had). You could extend to further drives by connecting to a floppy expansion port.
The original IIc ROM version (version 255) supported booting from external drives by simply entering
(Apparently, this was also documented in the manuals, and apparently, it only ever worked for ProDOS, the standard that came with the computer, but still better than nothing)
The follow-up ROM version (version 0, came in a ROM double the size of the original, so any omissions can't have been for space reasons), removed this feature, severely hampering usability of externally connected drives. This feature was never restored in any of the later ROM revisions (3, 4) until just recently in ROM version 4X, a hobbyist's project, which nicely presents a boot menu including an option to boot from external drives on CTRL-[closed Apple]-Reset.
As a work-around for the missing feature in later ROM revisions, Apple proposed (in the IIc Technical Reference Manual) using the built-in monitor and entering and calling a 10-bytes code sequence to boot from external drives (not exactly an improvement over
Why did Apple remove such an important feature? How was this accepted by customers? Why did they never restore that feature by adding the 10 missing bytes to a ROM that had by far enough space?
EDIT: The 3.5 ROM update manual bluntly claims
PR #7(from BASIC) or
7<CONTROL>P(from Monitor) no longer causes the system to boot from the external Disk II drive because that ROM space is now used for new features.
The removal I can confirm but the reasoning seems untrue to me (considering the size of the new ROM).
The IIc Technical Notes blatantly state
ROM Version 255 (the "correct" behavior)
PR #7 boots the second drive
ROM Version 0
PR #7 returns the message "Apple Talk Off Line"
(With no Apple Talk support on the IIc ever this does not look like a very useful message, but rather like a bug)
ROM Version 3
PR #7 kills the system
(Similarly unhelpful, but rather looks like another bug)