It's a great topic :) and with so many interesting ideas and constructions. In my comment I was trying to make the more general statement that is (likely) impo

It's a great topic :) and with so many interesting ideas and constructions. In my comment I was trying to make the more general statement that is (likely) impossible to achieve strong metadata privacy (or unobservable communication) without cover traffic. However, metadata privacy is a bit of a spectrum (see my other comment above). Adding latency at the relays/mixes already helps to make it harder for an adversary to correlate the traffic as their measurements get more noisy. This then very quickly gets to a level where we have "practically unobservable communication" as the signal to noise ratio is very low. By increasing/decreasing the latency of the intermediate hops one can directly control the trade-off between latency and anonymity. Check-out e.g. Figure 6 in the Loopix paper (https://www.usenix.org/system/files/conference/usenixsecurity17/sec17-piotrowska.pdf https://www.usenix.org/system/files/conference/usenixsecurit...). Similarly, this holds for cover traffic. In an ideal scenario the rate of the outgoing traffic is fully independent of whether one communicates or not. In a less ideal scenario we just add some other requests on top (e.g. a random browser bot on Tor) to create noise. In some cases this might be good enough. However, it won't satisfy any theoretical definition of unobservable communication which indicates that an observer can mitigate the noise by extending their observation over a long-long time.