It's a major part of the lawsuit itself.

I did not set out with the goal of suing Valve, but I have personally experienced the conduct described in the complaint. When new video game stores were opening that charged much lower commissions than Valve, I decided that I would provide my game "Overgrowth" at a lower price to take advantage of the lower commission rates. I intended to write a blog post about the results.

But when I asked Valve about this plan, they replied that they would remove Overgrowth from Steam if I allowed it to be sold at a lower price anywhere, even from my own website without Steam keys and without Steam’s DRM. This would make it impossible for me, or any game developer, to determine whether or not Steam is earning their commission. I believe that other developers who charged lower prices on other stores have been contacted by Valve, telling them that their games will be removed from Steam if they did not raise their prices on competing stores.

http://blog.wolfire.com/2021/05/Regarding-the-Valve-class-action

So before saying they are wrong, see what happens in the lawsuit alleging just that