Is he considered a disbeliever, the one who believes that Prophet Muhammad (peace and blessings of Allah be upon him) committed an act of indecency before his prophethood?

Whoever claims that the Messenger of Allah, may Allah bless him and grant him peace, committed indecency before his prophethood has slandered him and lied about him. Slandering the Messenger of Allah, may Allah bless him and grant him peace, or belittling him, is an act of disbelief by consensus. However, declaring a specific person an apostate requires the fulfillment of conditions and the absence of impediments.