You still can. I do it all the time.
It’s entirely possible that I’ve missed more recent legislation, so take this with a grain of salt. Canada has a “blank media tax” courtesy of the record lobby back in the recording tape days. There was much pushback from consumers when that fee was applied to things like video tapes, recordable CDs, hard drives, etc, but still exists as far as I know.
The recording industry was pushing for laws more in line with other jurisdictions, primarily the US. The government was open to it, but would then abolish the fees on blank media. Industry backed down because they get more from that fee distribution than they would ever get by having more restrictions. Of course, that doesn’t stop them from trying to shame us or blow smoke up our asses.
That means we are already paying a licence fee allowing us to copy recorded or broadcast material for personal use. “Personal use” is defined by what it’s not: rebroadcast, playing for the general public, and reselling. Thus, making a strictly personal copy is fine, as is making a copy for a friend, copying from an original you’ve borrowed (from a friend or from the library), recording legal broadcasts (like from radio, etc), and recording concerts unless the terms of admission expressly forbid it, etc.
This sounds like just standard traffic analysis. Nothing to do with WhatsApp or any other messaging platform. It’s been in use since at least WWII.
Who is talking to whom? How often? Under what circumstances? How do patterns of communication correlate with events? Who are the hubs of communication (ie leaders)?
The big difference between then and now is that instead of needing rooms full of people drawing graphs by hand, there is software to handle it. In turn, that means it’s not really important to have initial suspects to get started, because the computers are quite happy to tease out interesting signals from total communications. That also increases the likelihood of false positives, but the kinds of people who do traffic analysis at this level aren’t usually the kinds of people who worry about a little collateral damage.
It seems like a pretty tall order to construct a system of communication that is useful for coordinating activities, affordable to operate, and secure against traffic analysis. At best, you’ll end up back in a situation where other intelligence will be required to identify a manageable pool of suspects.