I opened a ticket with support, and after speaking to a network engineer the agent said that I could only get a single static IPv6 address (one-off cost of $14.95) and that I would need to switch to a business account if I "require a subnet or more than one(1) IPv6 address".

From reading the forum it seems that you could still get a /64 prefix and /56 delegated prefix in 2021.
There is a post from 2020 that also mentioned getting a similar response from support, which is why I want to double-check if Voyager no longer allows it or if I'm just unlucky.