Conventional wisdom based on third hand rumors of Strunk and White say that you should write with verbs and nouns, not with adjective and adverbs (well Strunkandwhite does say that!) that you should avoid using the verb "to be" as much as possible. I've bolded all the forms of the verb to be and all the adjectives here. Aside from to be, Frost uses a few other verbs, like hear, make, say, go, come, name, cease, sing know, and frame. He doesn't avoid adjectives, though there aren't adverbs obvious to me at first glance. He uses the verb be exactly six times in a sonnet of 14 lines, and about 11 or twelve adjectives. It depends whether you count nouns modifying other nouns like "midsummer" as adjectives or not. If you read the poem without knowing there was a dumb prejudice against this humble and useful verb, you'd think it wonderful. If you read it with the prejudice in mind, you'd still think it's wonderful and the prejudice stupid. Even if you only count clearly descriptive adjectives there is no sign that the poet was avoiding them, since he can't write four lines in a row without one.
THERE is a singer everyone has heard,
Loud, a mid-summer and a mid-wood bird,
Who makes the solid tree trunks sound again.
He says that leaves are old and that for flowers
Mid-summer is to spring as one to ten.
He says the early petal-fall is past
When pear and cherry bloom went down in showers
On sunny days a moment overcast;
And comes that other fall we name the fall.
He says the highway dust is over all.
The bird would cease and be as other birds
But that he knows in singing not to sing.
The question that he frames in all but words
Is what to make of a diminished thing.