Old English was not static, and its usage covered a period of 700 years, from the Anglo-Saxon settlement of Britain in the 5th century to the late 11th century, some time after the Norman invasion. While indicating that the establishment of dates is an arbitrary process, Albert Baugh dates Old English from 450 to 1150, a period of full inflections, a synthetic language. Perhaps around 85 per cent of Old English words are no longer in use, but those that survived, to be sure, are basic elements of Modern English vocabulary.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): About what percentage of Old English words are not present in Modern English?
85