No, loading is not a myth.
With high-inductance MMs and MI cartridges, loading is important to equalize the audible part of the cartridge signal.
However, when it comes to low-inductance MC cartridges, loading will not have a direct effect on the audible part of the cartridge signal, but will only affect ultrasonics and the RF region.
For low-inductance MC cartridges, loading is predominately a phono amplifier issue rather than a cartridge issue, but can have an indirect effect on the sound through mechanisms such as intermodulation.
With high-inductance MMs and MI cartridges, loading is important to equalize the audible part of the cartridge signal.
However, when it comes to low-inductance MC cartridges, loading will not have a direct effect on the audible part of the cartridge signal, but will only affect ultrasonics and the RF region.
For low-inductance MC cartridges, loading is predominately a phono amplifier issue rather than a cartridge issue, but can have an indirect effect on the sound through mechanisms such as intermodulation.