When the Shadowlands was first announced one of the first questions players had was will we be seeing the previous Lich King Arthas Menethil again inside of the Shadowlands.
While there have been minor comments from developers, the general feeling is if we do see Arthas at some point in the story he will only play a minor role and won't interact with any important living lore characters.
This is Blizzard we are talking about, however, so things may change in the future.
Will We See Arthas In The Shadowlands And What Role Will He Play
Patch 9.1 Chains of Domination is likely to bring a new chapter to the story of Sylvanas, and Arthas could potentially play a role in that.
Jaina Proudmoore is also one of the main characters in the Shadowlands whose relationship with Arthas may provide some reasoning to bring the character back.
And of course, the story of the Jailer continues as Zovaal carries out his plan alongside the corrupted Anduin.
Now we know that Arthas does not appear in 9.1, we can only assume if he appears it will be a part of a later patch.
We know that Arthas was sent into the Maw thanks to the Uther cinematic, so he would need to appear there or inside Torghast unless he found a way to escape, but Blizzard was heavy on the Maw being the inescapable location of death.
If he was to appear briefly with no story implications, many would assume that he would have appeared much earlier in the story, but since there is nothing yet maybe there are plans to introduce him someway into the story.
Many players do not want to see Arthas brought back as his story had closed and he deserves to have his ending as one of the most iconic Warcraft characters.