Hey Hey Yu, As I know in computer programming strings are written 0s and 1s. :) That was a sentence from a computer programming article?

This is a stylistic choice. Nothing is omitted, it is to avoid the risk of being misread. 'S' is the SI unit for seconds. 1s could mean 1 second, for example. If someone is reading it quickly they could also see it as 'Is.' 1s is also an electron orbital in chemistry. '0s' could be misread as 'Os.' It's not incorrect to write it without an apostrophe though. If you read a few style guides you can come to your own conclusions as to what way you prefer.