I am unable to find the endless mode in PVZ 2. Did they remove it in favor of the turn based challenge mode, or can it be unlocked somehow?
There is no endless mode per se, just the challenge mode. See this thread for example.
the Pyramid of doom, dead man's booty and Big bad butte are this games endless... They do in theory go on forever, but the plants get wiped and the map resets(and sun) every round!