I am familiar with the Germanic letter Thorn (Þ) and how it came to be written or printed as Y. My question is was this phenomenon responsible for the creation of the modern article "YOU" and if so when did it become common and when did people start using YOU over Thou?
No, Modern English's only second person pronoun was originally the second person plural. Old English and forms of Middle English maintained both plural and singular forms of the second person plural, as is and was common in Indo-European languages. "You" is the objective case of the Middle English plural pronoun "ye." "Thou" was the subjective case of the singular second person pronoun (objective case in "thee"). Both have easy PIE derivations from PIE *yūs and *túh₂, and cognates not only in Germanic languages (c.f. Modern German du and sie and Proto-Germanic *izwiz and *þū) but also with Indo-European languages as a whole (French tu/vous, Latin tu/vos, Greek σύ/ὑμεῖς). As in French, the English plural second person was frequently used in Middle English as a formal singular, and eventually came to displace the singular form entirely. "You" and "thou" have always been separate pronouns, and in grammatical function technically are not identical