This is a very interesting question the answer to which involves a discussion of negation in English and present participles. First let's look at negation in English. The negator never is an inherently negative adverb that may function as a constituent negator. This means it is always negative and may negate a single part of a sentence in addition to its function of negating a whole sentence: e.g., (constituent negation) I think he never sings; (sentence negation) Never carry antique chairs by the rungs. Never may precede the verb: She never cooks fish. Never may follow the verb: She is never coming.
Now to Keats. Keats uses poetic variation in positioning the negating adverb never to optimize his meter and in order to emphasize his dramatic point relating to the poetry of earth. Keats positions never behind the present participle ceasing rather than behind the to be linking verb is, as in "She is never coming." The present -ing participle ceasing comprises part of the compound present progressive verb (also called continuous progressive verb) is ceasing (i.e., simple present to be + -ing participle).
According to current Standard English (both U.S.A. and U.K.), the negator never would be placed following is and would be the constituent negator in the compound verb. In other words, negative never in current standardization would follow the linking verb and precede the present participle, producing is never ceasing.
It is interesting to note however that standardization for placement of this negating adverb never did not occur until sometime during or after the nineteenth century. In Keats' time, it was still commonly acceptable in varius English dialects to locate never in phrase-final position in the same pattern that you might use for a non-negating adverb, as in "He is cleaning carefully" or "She is sleeping soundly." In summary, the significant thing about the wording in this quotation is that the negating adverb never is in a dialectical phrase-final position instead of in the standardized position following the to be verb. The reason for the variation in placement is because it heightens poetic effect and meaning.