To be fair, although I don't write down the squawks and QNHs in the air now, I did do so throughout my training and earlier on in my flying career.
I don't know when it happened, but it was probably at around the 100-125 hour mark, and I wasn't doing it deliberately, it just happened, I think as a result of becoming more familiar with what to expect on the radio.
What also helps is push button transponders rather than the rotary knob types. I find myself keying in the squawk directly as I read it back. Fannying around with the knobs is more time consuming and doesn't lend itself to simultaneous reading back.
On the ground I do write down complex clearances especially at unfamiliar airports. As often as not I have to refer to the plate in those circumstances when they give a specific taxiway and holding point that I'm unfamiliar with.
Only a couple of weeks ago I was given the wrong taxiway, holding point and runway by an ATCO running on autopilot. As the wind was calm, I saw no reason to question it at the time, so I wrote it down, and read it back while writing it down. On taxying, I saw other traffic and questioned the instructions, glad I did... and I had the evidence.