What the heck does "anachronistic diction" mean? "Diction" is the way someone talks, and "anachronistic" refers to something that belongs to another time. If some girl were to show up to class in all seriousness wearing an enormous sweatshirt, a sideways ponytail, nylon leggings, and an old-fashioned walkman, you’d be like, "Whoa, there, how did you escape from the 1980s?" That is, her taste in clothing is anachronistic.
It’s hard to tell that Keats is using old-fashioned diction, because this poem was published in 1820, and everything from that period sounds old to us. But, trust us, the poem’s language is intended to sound a bit fancy. For example, no one from 1820 would have used "thee" and "thou" so many times. Keats was far from the only poet to do this, but he takes "old-school" to a new level.