I was curious whether .jsonl or .ndjson is used more often. Based on frequency in GitHub it looks like both are popular but .jsonl enjoys slightly wider usage:

  extension:jsonl   62K results [1]
  extension:ndjson  41K results [2]

[1]: https://github.com/search?q=extension:jsonl

[2]: https://github.com/search?q=extension:ndjson