Answered by Shaykh Sohail Hanif, SunniPath Academy Teacher
Assalamu alaykum
It is prohibitively disliked (makruh tahriman) to recite the suras of the Qur’an in reverse order consciously in obligatory prayers, though not in voluntary prayers. [Ibn Abidin, Radd al-Muhtar, quoting Tahtawi] If one began, unconsciously, one continues and it does not affect the soundness of one’s prayer. [Haskafi, Durr; ibid.]
However there is no such dislike to memorise the Qur’an in reverse order; in fact it is a very wide spread practice. [Ibn Abidin, Radd al-Muhtar, quoting Tahtawi]
However if one is practising what one has memorised, one should recite the suras in the order that they appear in the Qur’an.