When did Audacity end support of m4a files?
None of the published methods for getting Audacity to import m4a files work any longer.
I am forced to use someone else’s software.
M4A is a “container format” and may contain any number of audio, video, and even subtitle streams (https://en.wikipedia.org/wiki/MPEG-4_Part_14 and https://en.wikipedia.org/wiki/Digital_container_format). M4A also supports DRM (https://en.wikipedia.org/wiki/Digital_rights_management)
Audacity does not support DRM (never has, and probably never will).
Most other M4A audio files are supported by Audacity if FFmpeg is installed (see: https://manual.audacityteam.org/man/faq_installation_and_plug_ins.html#ffdown)