Use 3.5mm jack as line in for a musical instrument
I would like to plug in a 3.5 mm cable to the output jack of the electronic drumset and connect the other end to my computer to record the drumset's sound.... This was possible with my very old 2009 white MacBook. Can this be done in the newer models?


You can’t.  The audio jacks on all Macs are microphone inputs and not line inputs. Your 2009 MacBook had only a microphone input and not a line input.

Microphone versus Line Inputs

The key differentiation between mic and line signals is the voltage.  Microphone voltage is extremely tiny (millivolts) where line level input is about 1 1/4 volts.  Macs aren’t recording/pre-amp devices and do not contain the necessary components to raise the voltage to line level.  

The thread you referenced is about the TRRS jack or Tip, Ring, Ring, Sleeve and these are typically used in telco, not pro audio (yes, you can get very good quality sound from them, but they’re never found in even the smallest recording studio).  The cable, was just a "breakout" that separated the headphone (out) from the microphone (in) connectors.  It never converted a signal to line level.


  I realize the proper way of doing this would be to get an audio
  interface, but I don't want to invest in one just yet.


Yes.  However a simple audio mixer that handles mic, line, and instrument (what you need for your drum kit) that also includes a monitor port can be had quite inexpensively.