Previously if you had a dispute with Steam you would go to a mediator appointed Valve and you discuss things with Valve and come to an agreement. Sounds good, as it doesn't mean you have to involve lawyers.
Until you realise, Valve is paying for the mediator so the mediator has an incentive to agree with Valve to keep the business. In addition, any agreement is purely between you and Valve. It effects no one else, any previous agreement between Valve and the previous person has no bearing.

Contrast that to going to court, court decisions are binding and are able to be used in other court proceedings.

forced arbitration means you can't go through the normal court system if you want to sue them, instead resolving it through a private neutral third party.

and before, the waiver meant that you gave up your right to sue them in a class action lawsuit

getting rid of those is a massive W

Without getting into the weeds of arbitration—if you want to sue Valve for some reason, you now have to file in King county, Washington. This makes it too expensive to be worth it for any amount less than the cost of flying to and staying in Seattle for a lawsuit. Even if you're right and Valve is wrong.

Amazon recently did this too and it worked out well for them I guess, since other companies seem to have followed suit.