You are probably referring to Crin's article about measurements
Again, I can only speak from a personal experience of using my IEC-60318-4 compatible measurement setup and a few hundred measurements I collected where only some IEMs exhibit an 8kHz peak which I can hear and confirm by doing a sine-sweep.
I'm a strong believer in trusting your ears, not your eyes when listening and analyzing the sound. But I also like to take measurements afterward, to sync up and confirm what I'm hearing. I can't tell just by listening there is an exact "8kHz" peak. But I can tell there is a peak somewhere in mid-treble that irritates my ears
The measurement and sine-sweep confirm the exact frequency that bothers me. Of course, not everybody hears the same, just like not everybody has the same ear sensitivity or even the ability to hear high frequencies the same. But to my ears, using either foam or hybrid foam/silicone eartips did help significantly with the planar IEMs I tested.