Learn Spanish (Latin America): Antiguamente, los vecinos solían conversar en la acera. - In the past, neighbors used to converse on the sidewalk.

Both are grammatically correct and translate to "used to converse," but using the imperfect of soler followed by an infinitive puts special emphasis on the action being a long-standing habit or custom. While conversaban can mean "they used to converse" or "they were conversing," solían conversar strictly means "they were in the habit of conversing."

Can soler be used in the preterite tense, like solieron?

You will almost never see soler used in the preterite. It is almost exclusively used in the present tense for current habits (meaning "to usually do something") or the imperfect tense for past habits. Because a habit naturally implies an ongoing action without a defined beginning or end, the imperfect solían is the correct fit here.

Antiguamente translates to "in the past", but doesn't it literally mean "anciently"?

Literally, yes, because it is built from the adjective antiguo (old or ancient). However, in everyday Spanish, antiguamente is a very standard way to say "in the past," "formerly," or "back in the old days." It doesn't necessarily mean centuries ago; it just contrasts how things used to be with how they are now.

Is acera the standard word for "sidewalk" in Latin America?