The offset classes have been replace by margin classes. These are prefixed mr- for margin-right and ml- for margin-left.
The documentation demonstrates the use of ml-*-auto (where * is the target resolution -- eg lg or md.) This is the equivalent of setting margin-left: auto in your CSS, which is effectively saying "push this as far left as you can." By using both ml-*-auto and mr-*-auto, you can effectively center your columns.
This is the full list of prefixes for the auto property:
ml- margin left
mr- margin right
mb- margin bottom
mt- margin top
mx- horizontal margin (margin left + margin right)
my- vertical margin (margin top + margin bottom)
m- all margins
In addition to auto, you can specify the column widths -- ml-lg-2, etc.
So col-offset-2, which (I think) would shove the content left two spaces is equivalent to something like ml-2 or ml-lg-2.