Is it permissible to combine the Dhuhr and Asr prayers, or the Maghrib and Isha prayers, due to the excuse of major ritual impurity (janabah)?

It is not clear from the question what is meant by "excuse of janabah (ritual impurity following sexual activity or seminal emission)," but generally, janabah is not among the excuses that permit combining prayers. Praying on time is a very important matter, and Allah Almighty has warned those who neglect it, as He says: ﴿Then, there succeeded them a generation who neglected prayers and pursued desires; so they are going to meet evil.﴾ [Maryam:59]. It is not permissible to delay prayer beyond its prescribed time due to lack of purity, which includes delaying the ghusl (ritual bath) from janabah such that the prayer is not performed within its time. Al-Akhdari al-Maliki said: "It is not permissible to delay prayer due to lack of purity, and whoever does so has disobeyed his Lord."