Andrew Kent, a Professor at Fordham University Law School, argues that Israel is not obligated to accept a Palestinian right of return, as international law at the time the 1948 Palestinian exodus occurred did not render Israeli actions illegal, with documents cited by proponents of the right of return such as the Fourth Geneva Convention and the International Covenant on Civil and Political Rights coming into force after the Palestinian exodus had taken place.
Kent argues that these documents do not apply, as international law almost never applies retroactively.
https://ir.lawnet.fordham.edu/cgi/viewcontent.cgi?article=1498&context=faculty_scholarship