Old cellphones versus new cellphones noise
I'll give one answer. Any cellphone will have its transmit power output set by the local mast it is talking to - this is to avoid wasting too much battery power and also to prevent the sidebands of a higher power signal slightly overlapping (this was before GSM i.e. the old analogue days where each phone was allocated a frequency channel rather than a timeslot). Maintaining the power at an adequate level also means that you are not trying to login to several cells at once.

With the development of the cell network cells become smaller thus power outputs from handsets reduce thus interference with the input to an audio amplifier feeding a loudspeaker drops.

So, my single answer is the reduction in size of a cell. I live quite far away from my local cell mast and I still get interference thru my yamaha electric piano.

Your new phone may also be using a higher frequency band and the effectivity of RF/EMI blocking components on analogue inputs becomes better.