Is it true that reciting Surat Al-Qadr immediately after ablution atones for forty years of sins, and is there any proof for that?

There is no legal basis for reciting Surat Al-Qadr after ablution, and nothing in the Prophetic Sunnah indicates that. Indeed, some scholars have mentioned that it has no basis. There are authentic supplications and remembrances to be recited after ablution, and a Muslim must be diligent in observing them without adding to them.