The pilots have radio communication with each other while still in hyperspace. This has never been shown to be possible before, and in fact was a thing EU writers were told was not possible.
EU no longer exists in canon, so what the writers were told doesn't matter.
Besides, not being able to communicate AT ALL while on the move is ridiculous. Highly advanced military craft should have comms abilities at all times, barring localized phenomena. Saying they can't is as stupid as saying that modern aircraft can't communicate while over the ocean. Even modern 18 wheelers have had CB radio to communicate for the last seventy years. The simple fact is that hyperspace travel has been around for a LONG time, way longer than the 50-60 years spanning Episode I to Episode VII (you'd HAVE to have it available for everyone from military down to the space-truckers and family vacationers, otherwise there's no way they'd be able to sustain a galactic-scale civilization). If they didn't have a way to communicate, there'd be no way to warn anyone about bad things or potential hazards in transit.
Frankly, I'm glad they tossed that particular piece of rubbish out.
At a guess, I'd bet that the "rule" originated somewhere with West End Games and the Star Wars RPG. Probably something so that players had some kind of imposed limitation on them for communication purposes in hyperspace because it made for better gaming and still kinda made sense from a "physics" perspective. On the other hand, we're dealing with space fantasy/adventure, so there's no reason that you can't have "hyperwave communications" or something, kind of the same way you have sublight engines and hyperdrives as different things. So you could have sublight comms and hyper-comms, and then you create limitations by using a "They knocked out our hyper-comms! Oh no!" kind of scenario.
On balance, I don't think it matters a ton. Hell, they could work in "hyper-comms" now and say that the lack of hyper-comms in past years was a real weakness that engineers at Incom recognized and decided to work around. Or something.
Given that according to the EU, hyperspace travel had been around for twenty-five millenia (somewhat supported by Obi-Wan's statement that the Jedi were a galactic-scale organization for over a thousand generations) and that technology had most definitely not stagnated during that time, I find it ludicrous that in all that time, nobody had managed to crack a relatively simple communications issue. I mean, they can communicate across the galaxy in practically real time using sophisticated holographic technology, but they can't do simple voice communications with ships in hyperspace? Even the Federation in Star Trek managed to crack that, and they are both much younger than the Star Wars galaxy AND they travel through a different dimension.
Like I said, on the "physics" side, it sorta makes sense. You have everyone traveling in some alternate dimension, but it's not necessarily like hyperspace on, say, Babylon-5, where you're all in the same plane of existence together. Or at least, not necessarily. We've never really watched what happens with hyperspace travel and other ships. All we've seen is the Doctor Who time stream from the cockpit of the falcon, and otherwise a ton of ships all popping in and out of regular space. So, while it sort of makes sense to assume everyone is traveling in "parallel" dimensions, it also makes equal sense to treat hyperspace as, like, another plane of existence in which people can travel side by side in different ships.
I mean, it's hyperspace. We can make the rules up as we like, as long as we don't change them mid-story. And aside from the EU (now eliminated unless otherwise stated), we've never had any real discussion in the films about the "rules" of hyperspace.
