Notwithstanding the iniquity of Israel and the judgments that should come upon them, God would have compassion upon them for his Name's sake, and would appear for their vindication and defense. The "this" in Deuteronomy 32:34 is by some understood of the sinful doings of the Israelites which God should not forget or overlook.
So the Targum of Onkelos: "Are not all their works manifest before me, kept against the day of judgment in my treasures?" So also Calvin, "Quanquam de poenis hunc versum quidam exponunt, acsi Deus assereret diversas earum species apud se paratas esse, quas depromat quoties libuerit: rectius tamen est de sceleribus intelligere."
But there is a more 'comprehensive reference here. Not only the deeds of the transgressors, but the judgments that should come on Israel, and also God's interposition on their behalf, were laid up in store with him, and sealed up among his treasures.
All that had been done had been noted, and all that should happen was decreed, and should certainly come to pass. The "this' has thus both a retrospective and a prospective reference; it includes both the sin of the nation and God's dealing with them afterwards, as well as his judgments on their enemies.