The phrase "although it was once" is correct and usable in written English.
It can be used to introduce a contrast or a change in circumstances, often referring to a previous state or condition that has since changed.
Example: "Although it was once a thriving town, it has now become largely abandoned."
Alternatives: "even though it used to be" or "despite having been".
They have not seen a red-backed shrike before: although it was once common in southern Britain, its numbers went into freefall during the middle of the 20th century.
Although it was once a center of the slave trade, its origins lie partly in a remarkably utopian 18th-century settlement of emancipated slaves.
Although it was once New Jersey's insurer of last resort, it ceased performing that role in 1992.
It's a real paradise, although it was once a hellish prison island where political prisoners were held.
Although it was once thought to be a feature of Greek classical drama, it is now believed that it did not originate until the Hellenistic age.
Club fitting has been around for decades, although it was once mostly trial and error, making it so costly it was reserved for elite players.
And although its audiences had dwindled, it was once an anchor of the neighborhood.
I've yet to use it, although I was once tempted to on a drizzly day in the Great Dismal Swamp in Virginia when I was tired of paddling down a dark canal and thought I might call a friend, not for a helicopter rescue, but to be saved from boredom and the dreary task of paddling when nature's legendary wonders were not on view.
Although it's only once every few years and all you've got to do is walk around the corner and tick a box – what's the big deal?
A Fox spokeswoman said it was, to the company's knowledge, the first time geocaching had been used to promote a movie, although it was used once to promote a horse racing track in Georgia.
Although it was claimed that he was once engaged, Newton never married.
