Were those plugins included in the game of the year edition or have they always been separate? Are they needed for any mods? Where can I even get them?

No, they are not in the GoTY version.

You can get them on the wiki: http://www.uesp.net/wiki/Morrowind:Official_Add-Ons

Although you might want to use a modded version instead, because modders have cleaned them from a few small bugs.

I think that they are not included in GOTY version. And I do not remember any mods that would explicitly need them. If you want the original versions, you can get them http://uesp.net/wiki/Morrowind:Official_Add-Ons. But I strongly recommend picking some compilation that has bugfixes and other stuff. For example mine is http://download.fliggerty.com/download-13-1079. There was also even more comprehensive one in the works, but I cannot remember whether it was already released.

EDIT: Ninja'd

There are two mods that requires the official plugins.

http://mw.modhistory.com/download-35-5540

http://mw.modhistory.com/download-98-13853

I am using the Morrowind Overhaul- Sound and graphics mod, will these official plugins break anything?

Perhaps those plugins that's armor and the Bitter Coast Sounds plugin, but I'm not entirely sure about the Bitter Coast Sounds.

Well, besides mods that explicitly state that they are expansions of official plugins

It should work fine, I also use MGSO and official plugins and I have no problems.

Are you talking about my http://www.nexusmods.com/morrowind/mods/43931/??

It's the current-most up-to-date one. It was last updated in January, but hopefully I'll be updating it soon, if everything goes according to plan

Yes, exactly!