Ever implemented a DDR2 or DDR3 memory interface, or anything else that relies on correctly matching a high speed transmission line? The days of digital being just ones and zeroes ended a long time ago...
Digital is one and zero by definition, and always will be
There are two separate issues here. One is designing and implementing a system to allow it to work in a digital way. And then so long as you operate the system within the designed margins for digital operation, it then switches from being an "analog" system to a "digital" system. So in theory "digital designers" do not need to be concerned with the analog world, because someone with analog knowledge (who is not necessarily an analog design specialist) has designed that system to work for them.
But in practice, for that to be the case, you are no longer a "digital designer" but simply a "programmer". And programmers rightly should have no concern with the analog world.
There is no way anyone would call someone laying out a PCB, let alone high speed DDR, a "digital designer". You need at least rudimentary analog knowledge to lay out almost any PCB properly. A modern high speed DDR layout does not require a specialist analog design engineer though, but it does require a designer with enough basic knowledge of analog to understand things.
There have always been issues with designing digital systems, it's not just about speed.
For example, the early (non CMOS) digital systems had fanout current drive issues, even operating at 1Hz.
Similar issues still exist for example with FPGA "digital designers". You can't just use the one clock into your FPGA to drive unlimited numbers of internal gates in unlimited locations across the chip.
Basically, it's impossible to do any electronics design without a basic foundation of analog.
But one does not need to be an analog specialist for the majority of electronics design these days, but there are still a ton of niches were analog specialists are essential.
Dave.