Shalom Is there any specific halachic ruling used to forbid a man taking a woman as a pilegesh in the modern era or is it just that it has fallen out of custom? And if someone were to take a woman as a pilegesh wouldnit require a Ketubah and Get if they seperate as R’ Yaakov Emden zs”l seems to suggest? Thank you

ב"ה Shalom Your question is of far reaching consequences and it would require a treatise to cover the full scope of this subject. However, to give some perspective, we must immediately say, that the issue of Pilegesh is far from an issue which just has fallen out of custom, but rather one which is brought down in Shulchan Aruch (אבן העזר סי' כ"ו) who sees the option of Pilegesh as an absolute prohibition. The Rema who on the one hand quotes a more lenient position, also quotes a more severe opinion based on the Rambam, Tur, and Rosh, that this is an absolute Torah prohibition "There must not be any prostitutes among Israelite girls " (Devarim 23:18). The opinion of R’ Yaakov Emden, which you mentioned ( שו"ת שאילת יעב"ץ ח"ב סי' ט"ו ) is a lone opinion, and he himself makes the reservation that only with agreement of the "Gedolei Hador", the great Rabbis of the generation would he actualy allow his opinion to be applied. However, in the 200 years which have passed, no great Rabbis have agreed. Furthermore, even among the opinions who in certain situations did not find limitations from the Halachic point of view, they found moral reservations such as: it would lead to prostitution, promiscuity, and to the issur of niddah. (שו"ת הרשב"א המיוחסות להרמב"ן סימן רפ"ד) These things which were written more than 800 years are certainly relevant today. Therefore, a pilegesh cannot be allowed in our time. שו"ת וזאת ליהודה אבן העזר סימן ז All the best