stack size can indeed be unlimited. _STK_LIM is the default, _STK_LIM_MAX is something that differs per architecture, as can be seen from include/asm-generic/resource.h:
/*
* RLIMIT_STACK default maximum - some architectures override it:
*/
#ifndef _STK_LIM_MAX
# define _STK_LIM_MAX RLIM_INFINITY
#endif
As can be seen from this example generic value is infinite, where RLIM_INFINITY is, again, in generic case defined as:
/*
* SuS says limits have to be unsigned.
* Which makes a ton more sense anyway.
*
* Some architectures override this (for compatibility reasons):
*/
#ifndef RLIM_INFINITY
# define RLIM_INFINITY (~0UL)
#endif
So I guess the real answer is - stack size CAN be limited by some architecture, then unlimited stack trace will mean whatever _STK_LIM_MAX is defined to, and in case it's infinity - it is infinite. For details on what it means to set it to infinite and what implications it might have, refer to the other answer, it's way better than mine.