That is funny, I've found gravel bikes to be relatively compliant. But stiff carbon bikes of 10 years ago, or Aluminum bikes of 20 years ago may not be the best benchmark. ;-) Back in the day stiffness was where it was at.
These days bikes are more likely to be " "Laterally stiff and vertically compliant." To me, this means a stiff bottom bracket area that applies my power efficiently, but the fork, bars, seat stays, seat tube, seat stem can all be compliant, making the bike comfortable on the road.
Just with my bikes, my track and gravel bike have similar stiffness where it counts, and similar acceleration, but my track bike is a nightmare on anything less than velodrome smooth. I was shocked doing 20mph on some horrid washboard on my gravel bike with 32mm tires - and I was just cruising along without realizing how bad it was. None of my older bikes can do that.
I have some old bikes that really twist under power. I can see the crank arms and the bottom bracket twist, and if my tire is too big it will rub against the frame.
Disk brakes don't seem to be a friend of ride quality though. Those older forks with the J bend had suspension built into them. These days with straight blade forks, necessitated by the torque of disk brakes, require other creative solutions to get a little front end compliance in them.