hori873's avatar
TLDR: There's no mention of it leaving, and any way you look at it, it wouldn't make any sense not to be part of the Empire.

Simply the fact that there is no mention or confirmation that it ever left the Empire. In comparison, there are plenty of mentions of Hammerfell leaving, and Skyrim currently trying to (this one would be quite hard to miss), even for Black Marsh (which is named Argonia since the secession).
There would certainly be mentions of it in the nords' hate speeches, and I can't stress enough about just how important this point is.

Also: Morrowind hit rock bottom during the Red Year and the argonian invasion. It would be incredibly stupid to make a Brexit at such a moment, considering that it's economy, political power, and military power are close to nothing. The Empire, while it did recall it's legions back from Morrowind, would respond millitarily if the province tried to secced, and they would most certainly be sucessful (seeing that they can easily handle Skyrim with only a minor force, and Skyrim is a region unaffected by either natural disasters or war, not even the Great War).

Oh, and also there's Skyrim ceding Solstheim to Morrowind. Certainly they wouldn't have done it if Morrowind wasn't a part of the Empire, and even if they would, the Empire wouldn't allow it since any piece of land is just too important considering the ongoing cold war which they seem to be struggling very hard to wage. If Morrowind left the Empire after it took the island, it would give even more reason for the Empire to "pacify" the province.

Now the following is just my personal opinion but I think that Morrowind took an even greater hit than Pyandonea did in the War of the Isle of 3E 110.