but to hurriedly back away from the comparison of dictionaries and attempt to answer the question, it's all a matter of policies and standards. the editorial board (or perhaps the usage panel) will decide how to apply markings such as obsolete and archaic and rare and under what circumstances a word so classified will be dropped altogether.

a diachronic, or historical, dictionary such as OED will not soon drop any word; a synchonic dictionary such as W3 will be under constant revision in this regard.