An assassination rumor has constantly been repeated in the Russian language radio liberty service of the 1980s; In 1989 it was briefly discussed in Soviet literary magazines (1989: Москва №7, 1989: Слово №10, 1990: Молодая Гвардия №10) adding some hard arguments; f.e. reading some double-talk into his forensic medical examination (claims of a hushed up blunt force trauma). In 1992 the Russian Health Ministry ordered another repeated examination of his corpse which has repeated the original conclusion of 1925. If you can stomach the description of lofty poet like Yesenin in forensic terms click over here. In 1993 a special commission was formed in the Union of writers which add more some previously unknown details from the archives of the Leningrad police. It all points to your usual suicide. Still you do get the occasional paper defending the assassination thesis in the Russian literary press.

The conservative opinion in the Russian public right now is that Yesenin has indeed committed suicide. Historians don't care as much for poets but there is a discourse about the reliability of Soviet archives: evidently lots of stuff has been destroyed or never written down. On that you can read up in the Cambridge History of the Cold War.