Tax ID/social security number After leaving USA

Your SSN is yours for life.

Unless you close down your investment accounts before (or after) you leave the US, any income (interest, dividends, capital gains, etc) that is generated in the investment accounts is nonetheless taxable income in the US to you even though you are no longer residing in the US. Many countries have tax treaties or double-taxation avoidance agreements (DTAA) with the US, but these typically say things like each country agrees not to tax income generated in the other country, or each country agrees to give a credit on the taxpayer's tax return for the income tax paid to the other country, which effectively means that the taxpayer pays the higher of the taxes imposed by the two countries. Canada has a especially close relationship to the US, and so the rules might be a modification of the above, and of course, there will be another country that will be involved if the OP moves to another country (different from Canada and the US) upon leaving the US.