I'm 99.9% sure it is, but when I go to it's page, it says it's available in Gen 4 but requires Oak's Letter. Is this an Event Item, and if so, why does the page not say, requires Event Oak's Letter?
It is an event Pokemon, because Oak's Letter is an event item.
You could also get Shaymin with event distributions in DPPT, HGSS, BW, XY, ORAS, SU, and USUM. Also, the events for Shaymin are long gone. If you want Shaymin in any other game, you have to trade. You can also get Shaymin with a glitch in DP.
I hope I've helped!