It always surprises me that I've never seen anyone take real legal action against a platform like Steam over this sort of policy. You can put almost anything you want in your terms of service, but in most places if it's blatantly one-sided and unreasonable there's a chance that courts will strike it down, particularly in areas with relatively strong consumer protection laws like Europe. Something like blocking access to previous purchases because of an unrelated dispute seems about as one-sided and unreasonable as you could get; it's essentially no better than the kind of anticompetitive practices that are explicitly illegal in various other contexts.

I was also under the impression that the card networks frowned upon trying to prevent cardholders from exercising any chargeback right they have with their card. Then again, I have business interests that accept payments via these card networks and I've never even seen the full terms that we supposedly agreed to on the merchant side, so I don't know how I'd check even though we're presumably subject to the same rules, whatever they are.