Is it still a good idea to put non-breaking spaces in empty table cells or is that no longer necessary?
It is still a good idea for the 0.000000001% of instances where the cell needs to be empty. It certainly doesn’t hurt to do so although you could consider displaying the dates from past an future months in a different colour instead of empty cells in calendar tables, zero instead of empty cells down the diagonal of tables giving distances between locations, and whatever actual value is most appropriate in other data tables.
Empty cells in tables were most common back in the days when HTML tables were misused for page layout. Now that CSS tables can be used for page layout (where such a layout is required) the empty cells will not need to be defined at all.
Good point. Thanks.