The entire US banking system is built around debits, delayed settlement, and reversible transactions (as contrasted with bearer tokens, like cash). If a digital payment method doesn't support chargebacks/reversals, then it's not going to be exposed to consumers in its raw form. Rather it will have conventions layered on top of it that
do support these things - like your bank contacting the receiving bank and saying "this transaction was unauthorized, please send the money back".
I haven't read too much on FedNow, but I would expect this will take the form where accounts that can receive payments assent to their bank reversing transactions for various reasons (and then using another non-reversible FedNow transaction to send the money back). So at the consumer level, chargebacks will still exist (for better and for worse).