Whose expression is being curtailed? Everyone's. You can't, as an individual, enforce your specific set of opinions on the rest of the world. You can state your opinion, and if you feel strongly enough, attempt to convince others that your opinion is right. But that is where your freedom ends.
For example, the recent case of someone wanting to retcon/editorialise a published work that is problematic. It was then, it is problematic now. The solution, though, is not to erase it (or take steps that are tantamount to erasing it). The solution is to let your voice be heard. Do not try to redact or edit history.
Your other statement is a bit muddy though. Elaborate please.
This is amazingly wrong. The company is choosing to not write Dark Sun because of the slavery in it. Nobody is forcing them to not write it; their freedom of expression is not being hindered. Nor is there any erasure, because the older books, with all the slavery, are still for sale. You can get the first boxed set as a pdf on DM's Guild for $9.99. You can even buy a "gently used" boxed set on Amazon for $245.
Nobody is being silenced and nobody is being censored or erased. There's a DS subreddit with over six thousand people subscribed to it, talking about the game.
In fact you can, right now, write a Dark Sun ebook and release it for free on the Dark Sun subreddit, or make a Dark Sun blog and include as many slaves as you want. If you don't want to redact or edit history, why don't you do this?
You made affirmative claim about "only a small number of people are advocating for this", so that's on you. You don't get to ask @Irlo where they got their numbers and then suddenly make unproven statements for your argument.
Nobody's expression is being curtailed. Also, "freedom of expression" refers only to the government stopping people from expressing themselves. WotC is a private business. They can do whatever they want.
Now, if the actual government told WotC they weren't allowed to produce games with slavery in it, you'd have a point. But they haven't.