Complex sentence: All that remains is more and more precise measurement has been widely misattributed to Kelvin since the 1980s, either without citation or stating that it was made in an address to the British Association for the Advancement of Science (1900).
Split into simpler sentences: All that remains is more and more precise measurement is given in several sources, but without citation. It is reputed to be Kelvin's remark made in an address to the British Association for the Advancement of Science (1900).