It's largely an issue of the difference between contradicting existing information or revealing unknown information. The later is okay, mostly, but the former is an inexcusable use of the device.
Why is it inexcusable? Like many things, I think it has to come down to a cost and a payoff. The payoff of a retcon is being able to sidestep things in the past that were mistakes in hindsight, and the cost is suspension of disbelief. Somewhere it's implied: "Well, the story that was told years ago was wrong or misleading or should just be ignored, this is what's actually happening now" and it can be a little jarring, but can also be worth the break in immersion.
Anikin Skywalker was not actually dead like we were told in Episode IV. Time Turners somehow no longer exist in the Harry Potter universe from Goblet of Fire onward. Nissa Revane has a different past and personality than what we saw in previous seasons. I'm fine with those things, others think they're deal-breakers. "Inexcusable" sounds a bit absolute.
Do you honestly not see a difference between what Lucas did with Skywalker and what Wizards did with Nissa? In Episode IV, Ben Kenobi tells Luke that Darth Vader killed his father. This is second-hand information passed on by an unreliable narrator. The "retcon" was delivered by way of story development: Luke learns Darth is his father, and then confronts the ghost of Kenobi, who explains his reasoning for the deceit. If you need to do a retcon, that's how you do it. You fold it into the existing story. You don't just go back to Episode IV and say "No, Ben never said that."
With Nissa, we have stories and moments in time that exist in MTG canon. In those stories, Nissa has specific and defined character traits. The retcon has gone back and said, "yeah, that's not what happened. Something else happened now." On top of that, making Vader into Luke's father gave the story more direction and more emotional weight to it. Taking away Nissa's flaws and her motivation detracts from the story, because there is no character development available to her now.
If that had stuck with the originally laid-out character, they could have used it to show advancement. Rather than "retconing" it away, she could have grown out of it in a way that the reader could have actually seen. In a way, it goes back to the mantra in writing of "show, don't tell." Rather than telling us "Oh, Nissa's not a racist anymore." They could have shown us outgrowing her xenophobic attitude. More than that, by wiping out Nissa's established past, they are telling us that those stories and those moments don't matter, and didn't exist. Star Wars didn't do that. They gave a reason why we were presented what was presented, and maintained the validity of the original scene.
This. We barely even knew Darth VALER anyways, while we knew Nissa well.