Yes, they do if they are going to impose criminal punishment. The onus is on the crown to prove every element of the case. It doesn't matter if you contest it or not. If you plead not guilty then they have to prove each element of the case.
Here are the first few cases of personal tax evasion that come up on canlii when you search for tax evasion cases:
http://canlii.ca/t/21j7z
http://canlii.ca/t/fppg9
http://canlii.ca/t/1rkxs
None of these cases mention anyone searching for anyone's SIN or even mention the SIN at all. If you do believe that these cases do consider the question of whether the defendant had a SIN then please poit out the relevant passages.
If not, then these are just the first few cases that show up and they seem to confirm my sentiment that I have never seen a judge ever require proof that the accused had a SIN. Your claim that you have never seen a case that didn't raise this issue seems to make no sense.
Obviously if they wanted to charge you, the fact you didn't respond to a letter from them wouldn't make any difference. If they wanted to lay a charge they certainly could.
As you are now aware, there are many cases in which the issue of SIN is never mentioned at all in the decision. Also, take note that these decisions are specifically setting out the required elements of the offence and they do not mention having a SIN as being a required element. It is not mentioned at all in fact.
So would you say these judges must be corrupt because they are leaving out something that you claim to be an essential element of the offence?
Why not call them and ask them speifically about what you are doing. Ask them something like "do I have any obligation to report my earnings if I have chosen to no longer be associated with my SIN." I wonder what they would say?