> bank account and routing number, which while common in the EU, is extremely unusual and invasive in a US context
So now you're moving the goalposts and saying that it being extremely unusual precludes it as an option? Of course any alternative to the Mastercard/VISA duopoly is going to be unusual. By definition, since they are dominant, any other way of payment will have to be outside the norm.
> American Express ... wow, it's been years since I did this but I think it involves actually going to an Amex brick-and-mortar location, and so wouldn't actually count.
It does not require going in person at all. You can get qualified for a card online and they'll ship it to you.
> Bitcoin is equivalent to paying in a different currency, which while possible, mostly breaks the terms of the comparison
Again, by construction of the question, alternative payment schemes will necessarily be different. The other players here are PayPal, Discover, and AMEX.
The alternatives for Parler (going to another hosting provider than AWS, alternate App Store on Android, and no app on iOS) are at least as unusual compared to the alternatives for payment. Just like your Discover card won't be accepted at most places, or Bitcoin or PayPal, most users are not going to accept a modern day service that doesn't have an app. Or even more egregiously, in Stormfront's case, users are absolutely not going to learn how to use Tor just to access a site. Parler isn't there yet, but it may well be soon if their domain registrar drops them. Or even Level3 or BGP providers.