Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
Asking the Prophet ﷺ to make du‘a or seek forgiveness for a person after his death is not, in itself, shirk if the person means that the Prophet ﷺ should ask Allah on their behalf. However, it is not a prescribed practice and is considered an innovation (bid‘ah) that the Companions, may Allah be pleased with them, did not practice.
During the Prophet’s ﷺ lifetime, the Companions used to ask him to make du‘a and seek forgiveness for them. However, after his death, they did not go to his grave and say, “Pray to Allah for us,” or “Ask Allah to forgive us.”
One of the clearest proofs of this is what happened when the Muslims suffered from drought during the caliphate of ‘Umar ibn al-Khattab, may Allah be pleased with him. They did not go to the Prophet’s ﷺ grave to ask him to pray for rain. Instead, ‘Umar asked al-‘Abbas, may Allah be pleased with him, to make du‘a for them, saying:
“O Allah, we used to ask our Prophet to supplicate to You for rain, and You would give us rain. Now we ask our Prophet’s uncle to supplicate to You, so grant us rain.”
[Reported by al-Bukhari]
If asking the Prophet ﷺ to make du‘a after his death had been prescribed, the Companions would have been the first to do so.
If someone says, “O Messenger of Allah, ask Allah to forgive me,” intending that the Prophet ﷺ ask Allah for their forgiveness, then this is asking him to make du‘a after his death. It is an innovation and is not prescribed, but it is not, in itself, shirk.
However, directing acts of worship to someone other than Allah, or calling upon a created being in matters that only Allah has power over as an act of worship, is a different matter with its own ruling.
What is prescribed is for the Muslim to send greetings of peace upon the Prophet ﷺ and then direct their du‘a and request for forgiveness directly to Allah Almighty.
Allah says:
“When My servants ask you concerning Me, indeed I am near. I respond to the invocation of the supplicant when he calls upon Me.”
[Al-Baqarah 2:186]
Therefore, whoever previously asked the Prophet ﷺ to seek forgiveness for them after his death should stop doing so once they know the ruling and should ask Allah directly for forgiveness.
Asking the Prophet ﷺ to make du‘a after his death is an innovation that the Companions did not practice. It is not, in itself, shirk if the intention is that the Prophet ﷺ ask Allah on behalf of the person. What is prescribed for a Muslim is to direct du‘a and requests for forgiveness directly to Allah Almighty.
And Allah knows best.