Is it true that it was reported from Ibn Abbas that the Prophet (peace be upon him) would perform all prostrations of forgetfulness (sujud al-sahw) before the salām in the latter part of his life, or is this the saying of Al-Zuhri, and what is the ruling on that?

No statement has been found attributed to Ibn Abbas claiming that all prostrations of forgetfulness (sujud al-sahw) are performed before the salaam. The people of knowledge have deemed the hadith of Al-Zuhri, "And the last of the two matters is before the salaam," weak due to the weakness of one of its narrators and an interruption in its chain of transmission. The opinion that the prostration of forgetfulness after the salaam has been abrogated is not correct, as it is possible to reconcile the various texts on this matter. The hadiths regarding the prostration of forgetfulness, both before and after the salaam, have been reported in different scenarios, which allows for their reconciliation. It is best to conform to the Sunnah in instances where prostration is mentioned as being either before or after the salaam. Sheikh al-Islam Ibn Taymiyyah differentiated between addition, omission, and doubt, preferring that the prostration of forgetfulness be before the salaam except in two cases: performing the salaam after omitting one or more rak'ahs, and doubt accompanied by a strong presumption, in which case the prostration would be after the salaam.