Is Zakat obligatory on the monthly salary that reaches the nisab but is not spent until after years, even though it can be spent at any time?

If the salary reaches the nisab (minimum threshold) by itself, or by combining with money and trade goods, and a full year passes (lunar calendar), then its zakat becomes obligatory. If these conditions were met for the salary during the past years, then its zakat is binding upon you. The fact that you have not received it does not affect the obligation of zakat, because it is a debt owed to you by one who is solvent.