Are you asking who first began to decipher Old English texts in the Modern English speaking era? If so you may be interested in this page which gives some background on the discovery of Beowulf, one or the earliest written records of Old English. It was discovered in the mid 1500s, just as what we call Modern English was first being recognisable.

Otherwise could you please clarify the question?

Also a minor correction to your premise: It's not that OE literacy declined. It's that OE gradually developed into Middle English, just as modern written English today is different from written English of 100 years ago.