This case is about which sins may not be forgiven,
is claimed by some to have been abrogated by
Ibn Al-Jawzi reports this case in his book, "Nawaasikh Al-Qur'aan", as follows:
ذكر الآية الثالثة: قوله تعالى "بلى من كسب سيئة وأحاطت به خطيئته"، جمهور المفسرين على أن المراد بالسيئة الشرك فلا يتوجه على هذا القول نسخ أصلا وقد روى السدي عن أشياخه أن المراد بالسيئة الذنب من الذنوب التي وعد الله تعالى عليها النار فعلى هذا يتوجه النسخ بقوله "إن الله لا يغفر أن يشرك به ويغفر ما دون ذلك لمن يشاء". على أنه يجوز أن يحمل ذلك على من أتى السيئة مستحلا فلا يكون نسخا
Ibn Al-Jawzi says that the majority opinion is that the sin referenced in 2:81 is shirk and that makes it agree with 4:48 rather than be abrogated by it. He argues that it is possible that the sin referenced in 2:81 is the one committed with defiance and that is not forgiven.
I do not agree that the sin in question is "shirk", but I could be wrong. I see Al-Jawzi's argument reinforced by the verb كسب
(to earn) which implies that the sin is in defiance. I also see the clause وأحاطت به خطيئته
(and his sin surrounded him) further paints the picture about the kind of sin in question. It's the sin from which the person did not and would not repent until he died. In which case, that sin is not forgiven either. The fact that God says in 4:48 لمن يشاء
(to whomever He wills) means that God reserves the right to forgive anybody but He won't forgive the sin He described in 2:81. No abrogation needed.
Besides, these two verses are declarative and abrogation applies only to imperative sentences.