When Boxer finally succumbs to his poor health, what lie is spread about his care?

Boxer, the strong and loyal horse, is one of the most important members of the rebellion. He develops a codependent dynamic with Napoleon and dedicates himself to the cause. However, Boxer's enthusiasm and work ethic burn out in the end. Whilst rebuilding the windmill, Boxer's lung is damaged from his extreme work effort. Boxer eventually collapses from exhaustion and weakness.
Napoleon decides that Boxer is no longer a valuable asset to his rebellion and growing regime. Boxer, on the other hand, believes that he will be taken care of by Napoleon, whom he states is "always right." However, the naive Boxer does not realize that Napoleon does not reward loyalties and only manipulates animals to increase his power.
Squealer later tells the other animals that Boxer will be transported to a veterinary hospital in town for much-needed care. However, Benjamin, a good friend of Boxer's, read that the van meant to transport Boxer was owned by a slaughterhouse. Boxer also realized his true fate, but it was already too late, as he was unable to free himself from the van.
Later on, Squealer tries to quell the other animals' fury and distrust by explaining that the van was owned by the surgeon and that it was recently purchased from a slaughterhouse, hence the markings on the side of the van. This, of course, is a blatant lie.
Boxer's character and fate are an allegory into how Stalin manipulated the populace in Russia and then discarded them when they were no longer needed. The hardest workers were not rewarded in the end, because all of the rewards went to the dictator and top officials.
Squealer's lies are meant to represent the Stalinist propaganda machine, which tried to deceive the population through misinformation and blatant lies.