Every one of these symbols goes before the number: $€£¥₹

Where are you from where it comes after? What currency do you use?

The only time this comes after the number, that I am aware of, is when you use abbreviations like USD or EUR instead of the symbol.

Where are you from? In English it's always the case that you put the dollar sign before the number.

I understand that it’s different in other languages, but I often see this behavior from native US Americans who only speak English.

Hence the underline under the 00 cents/pence on a check/cheque.

Yes, it’s relevant in this day and age, because it’s still incorrect to do it the other way (at least in English and with $£¥ and a couple others, as another commenter pointed out).

When I read “20$” my first thought is that the person writing it didn’t pay attention in elementary school.

And being lenient about it on the web leads to the behavior leaking into the real world on things like shop signs.

Don’t even get me started on people who (mostly IRL) write things like “0.99¢” which makes me want to give them a cent and tell them to keep the change.