One of the pitfalls in lexical research is relying too much on dictionaries and glossaries and not enough on primary sources. Often, a term will be recorded in a glossary and then copied again and again over the years by other dictionaries, without any evidence that the term was ever widely used. You see this a lot with slang terms and murder of crows and other collective animal names are another case.
Many of the “fun” names for groups of animals, including murder, got their start in a handful of Middle English glossaries. They do not appear in any extant primary sources (literature, diaries, books on hunting, etc.) and were probably never commonly used. Copied from dictionary to dictionary over the years, they never appear to be actually used until the 20th century, when they were revived somewhat as “fun facts about language” and managed the occasional actual use by a real person. Even then, their use is very rare.
(Note that one of the older threads cited by Dr. T., the good doctor says that the OED doesn’t include this sense of murder. Since that thread was written, the OED has been updating the Ms and has included an entry for murder of crows, which Dr. T. cites above. Just want to make this clear for those reading the older threads and wondering how he could say it’s not in the OED in one place and then cite the OED in another.)