That is interesting. I knew roughly what his epitaph was but decided to look it up. That was the way it showed so I just copied it.
Anyway it is clear Shakespeare didn’t want anyone to mess with his grave.
I got curious. The image of his actual graven epitaph most certainly has Y, with letters appended in the exponent area.
I am assuming that this is a stylization during the period of transition away from use of thorn, eth, and ash (Æ and æ). The wikipedia page (a good source for anything beyond the ability of modern politics to corrupt) suggests that printing presses didn’t support the runes, and Y was often used as a substitute despite not being pronounced that way.
Shakespeare’s english would have had very little old norse influence, as the vikings never conquered wessex in the 9th century (they did conquer most of the rest of england). His plays still use the old english third-person singular -eth verb ending, rather than the -s in the rest of england. (telleth, rather than tells). He also, iirc, uses the old second-person sing. pronoun, Thou, thy, thine.
The english we speak today is more the london variant, which was heavily influenced by old norse, and even after all the influence french later had, still is iirc something like 10-15% words directly descended from old norse words that entered the language. It is also suggested by many philologists that we have the danelaw to thank for the almost total end of the old english declension system and possibly gender too (think german, etc.) We only have the genitive today.
http://en.wikipedia.org/wiki/Ye_%28pronoun%29