Learn Farsi: بناهای تاریخی رو به افول و فراموشی است. - Historic buildings are heading towards decline and oblivion.

Yes. The phrase رو به (heading towards / facing) governs the entire coordinated pair افول و فراموشی (decline and oblivion). This construction (رو به + abstract noun + copula) is a very common way in Farsi to express an ongoing trend or trajectory, much like saying something is "on the decline" in English.

The subject is بناهای تاریخی (historic buildings) which is plural. Why does the sentence end with the singular verb است?

In Farsi, inanimate plural subjects (like buildings, ideas, or objects) usually take a singular verb. While you could technically use the plural هستند, using the singular است is the standard, natural rule for non-human plurals.

The word for oblivion here is فراموشی. Is this built from the common word for "forgotten"?

Exactly! It is formed by taking the adjective فراموش (forgotten) and adding the suffix ی- (which creates abstract nouns). So it literally means "forgetfulness" or "oblivion". The رو به construction relies heavily on these types of derived abstract nouns to describe processes.

Could I just use normal verbs to say these buildings "are declining" instead of using the رو به + noun construction?

You could use a present continuous verb, but the رو به ... است construction sounds much more elegant. It is highly preferred in formal, journalistic, or written Farsi to describe gradual, long-term trends and shifts rather than immediate actions.