Mechanized farming reduced the number of on-the-farm jobs per acre.
The industrial revolution and subsequent improvements have reduced the number of worker-hours needed to make X number of widgets.
Automated telephone dialing greatly reduced the number of telephone operators per 10,000 telephone lines.
Automated telegraph repeaters greatly reduced the number of telegraph operators needed.
Voicemail reduced the number of corporate call-takers needed for a given number of incoming calls.
And so on.
But in the meantime, new kinds of work were created, and overall un- and under-employment in the USA at least has been at managable non-crisis levels for decades.