Here, for context, is Yevtushenko's "Conversation with an American Writer (1961), as presented on the PoemHunter Website:
Conversation with an American Writer
'You have courage,' they tell me.
It's not true. I was never courageous.
I simply felt it unbecoming
to stoop to the cowardice of my colleagues.
I've shaken no foundations.
I simply mocked at pretense and inflation.
Wrote articles. Scribbled no denunciations.
And tried to speak all on my mind.
Yes, I defended men of talent,
branding the hacks, the would-be writers.
But this, in general, we should always do;
and yet they keep stressing my courage,
Oh, our descendants will burn with bitter shame
to remember, when punishing vile acts,
that most peculiar time, when
plain honesty was labeled 'courage'...
The OP asks whether the sentence in bold type expresses optimism or pessimism, but I think that the answer to that question is rather tangential to the larger point of the poem. On one level, undoubtedly, "Conversation with an American Writer" looks to a better, freer future in the Soviet Union and considers how the people of that time are likely to view their predecessors who lived in a time of repression, fear, and moral compromise. To the extent that this future viewpoint presupposes a world in which people will be freer, more confident, and truer to their moral principles than they are in 1961, Yevtushenko is expressing optimism about the future.
But it seems to me that, in general, he is far less interested in the people of the future than in comparing himself—to his advantage—with his Russian contemporaries. In fact, he devotes most of the poem's space to inverting Isaac Newton's famous expression of humility with regard to his genius. "If I have seen further [than others], it is by standing on the shoulders of giants," Newton wrote in a letter to Robert Hooke in 1675. In Yevtushenko's telling, this assessment becomes something like "If I seem a giant among men, it is because I am surrounded by moral pygmies."
Yevtushenko doesn't deny that he is morally superior to his "colleagues" and that he refuses to "stoop to the cowardice" that they habitually display; he simply quibbles with his admirers' tendency to call his virtue courage rather than honesty. The poem is thus an exercise in sneaky self-aggrandizement—an extended humblebrag, as Oxford Dictionaries Online define that term, closing with a coda about how people in the future will recognize his true stature—not as colossus of bravery, but as an honest man in a nation of venal opportunists.
Yes, I'd say that Yevtushenko is pretty optimistic in the last part of the poem.