Use a contrasting colour to indicate hidden rows/columns
It is very hard to notice where hidden columns are at a quick glance. The boundary between the column labels where the hidden column is doesn't stand out. Please make this boundary stand out more, perhaps by using a contrasting colour. The same goes for hidden rows. This would help avoid silly mistakes involving deletion of row/column ranges that appear to be empty.
Excel 365 for Mac has a thick green line that indicates when a row/column has been hidden. I would like this 'feature' on Excel for Windows.
Tesh Vora commented
I endorse this. Further more - there should be an option to high-light the fact that there are hidden rows within the sheet for example using a horizontal ligthening bolt (with a further option to print this lightening bold)