Is there a way to add the ABC widget as a data screen in an activity?

You can define a Screen with altimeter, barometer and a compass gauge datafields. Or you can long press the Back button during the activity and simply go to the ABC widget. Widgets cannot be added to an activity unfortunately.

Alternative solution is using a Connect IQ datafield which has the functions you need.

This screen was available in the Fenix 3, 3 Hr, 5 and 5+.  For some reason they removed it in the Fenix 6.  I'd also like to have this ABC screen back again.

Ya, I'd just build my own with a custom 3-field datascreen. Graphical compass on top, Barometer in middle, Elevation on bottom.