So folks, what do you think? Is slavery still around these days in Morrowind, or was ist abolished after the argonian invasion with its massive death toll? Do the Dunmeri fear another Invasion if they rely on slaves again?
Or is the need of slavery 200 years after the Red year and the invasion bigger than ever (rebuilding, cheap labour costs in economically hard times, etc.pp.)?

I read somewhere (can't cite a source off the top of my head) that the king abolished slavery in Morrowind all together after the events of Morrowind.

Having not been through the Dragonborn content yet, I can't speak as to whether or not 200 years has seen an alteration in this policy. I doubt it though. After what the Argonians did to them last time, I can't see the Dunmer being stupid enough to risk antagonizing them again.

Then again, this is the Dunmer we're talking about.

True.
However, King Helseth was/is a Hlaalu, and Hlaalu was de facto overthrown and exiled from Morrowind. So with the driving force behind the abolishment of slavery gone, and House Redoran in charge (who started a civil war because of Helseth`s decision to renounce slavery)...

I still don't see them risking it in the wake of rebuilding, but again it's entirely possible they're using slave labor. I need to get off my lazy butt and get into the Dragonborn content.

The argonian invasion was 200 years ago and hit them in a weakned state without a standing army to speak of (just small house armies) and in the middle of volcanic disaster. I don`t see the Dunmer very afraid of the Argonians any more; at least there is no reason not to use khajiit slaves again.