The number of front panel buttons on AV equipment started to disappear around the early 1990’s as more emphasis was made on creating menu’s to help navigate the myriad of functionality that kept increasing (in particular on AV Receivers’ and VCRs’; not LD players’).
So the good news is that we never saw menus on LD players (except DVL units due to the DVD functions mainly) but the number of buttons did start to dwindle over time. The players with the most front panel buttons were the CLD-3030, CLD-3070, and possibly the CLD-3080 (mostly in that pop out drawer).
But getting to the point of this topic (sorry for being long winded as I like getting into history too much
) the audio buttons —audio monitor and digital/analog — were the rarities that never had buttons on the front panel at anytime from what I can remember unless we go back to the very early top loaders (which were analog only and thus would only have the audio monitor button).