I know there was still slavery in the late third Era until it was apparently outlawed by King Helseth, but is it known if it remained so? Two hundred years is a long time and the Dunmer are really picky about their traditions. Is there any information on whether slavery persisted afterwards? Or was it abolished completely?

Red Year and Argonians pillaging Morrowind really put a damper on the whole slave thing. I'd be very surprised if it was completely wiped out mind you, but its not exactly as attractive there as it once was.

You could make the case that the Thalmor are trying their hand at the whole slave/indentured servitude angle. The Bosmer and Khajiit seem to have gotten the shaft in a lot of ways, judging by the Thalmor embassy.

Hlaalu and Dres worked to abolish slavery in Morrowind prior to the Red Year and now after the Argonian sacking of the South and Red Year I doubt slavery is rampant in a rebuilding Morrowind where only Hlaalu has been kicked off the Council with Redoran holding the most power running Morrowind from Blacklight. I don't remember Redoran being pro-slavery in TES 3 but I may be wrong.

Wasn't Dres pro-slavery though?

They were which made no sense, House Dres' entire economy depended on the vast amounts of plantations they had and the entire slave-trade.

Some people theorize that Hlaalu forced/coerced Dres into abolishing slavery, possibly to appease the Imperials.

There's this discussion from The Infernal City:

"House Redoran employs them in a lesser degree, though some of their warriors use slaves like gladiators in ancient Rome, pitting them against beasts for entertainment"

http://uesp.net/wiki/Morrowind:Slavery

Redoran is pro-slavery in the TES 3

Eh. I was kinda hoping Redoran would employ large numbers of warrior slaves, as popular as it was across the Muslim world for a long time. Though I suppose Telvanni does this to an extent...kinda...

Let's not forget as well that House Hlaalu was run behind the scenes by the leader of the Camonna Tong, who is a well-known major slaveholder. I have no idea how Hlaalu got Dres to "give up" slavery, but I can easily imagine that there was something more sinister going on behind the scenes in both cases (although by that time one would assume that Helseth was not being explicitly influenced by the Camonna Tong, being something of an outsider to Morrowind politics, to say nothing of the strong political backing he already has from his mother and his own strength).