What’s the difference between terminators and separators here? The ndjson spec [0] doesn’t say anything like that, and it seems that ndjson and jsonlines are identical in what documents they accept.
You can detect and error out if you see an unterminated record at end of transmission. With separators, the producer might not put a separator after the last record, because there's nothing to separate it from there.
There's no justification for not using terminators, it's just a bad spec. Unsurprising story: the variant with better marketing has less technical chops.
i.e. a documemt without a newline at the end is valid jsonl, but invalid ndjson