For example, if you press grab 3 frames before shieldstun ends, that’s 1 frame outside the current 2-frame buffer window, so the character would shieldgrab 1 frame late after shieldstun ends (to a max of 6 frames before shieldstun ends, to match the old OoS input buffer). For fairness, this would also of course apply to the buffer for the attacker.
This system would still reward precision and keep mashing in check like the current 2-frame buffer, while also returning some of the fluidity and good game-feel that was lost when the buffer was decreased.