I have seen things ranging from Beria outright admitting it to other party members, to early drafts of the autopsy and medical records indicating poisoning. Is assassination considered plausible by historians or is this a debunked conspiracy theory?
More can always be said on the topic, but there are some previous write-ups on this topic which you might find interesting (which are both mostly focused on Beria):
How likely is it that Stalin was poisoned? by /u/sergey_romanov
How likely is it that Stalin was actually poisoned by one of his right hand men? by /u/jezreelite