After implementing fractional second parsing in and
ARROW-9107 and ARROW-10337, no support for inference of subsecond timestamps was added (see [code]([https://github.com/apache/arrow/blob/master/cpp/src/arrow/csv/inference_internal.h#L115), also mentioned in ARROW-3738).
The InferState logic could test for TimestampNS (with nanosecond resolution) after Timestamps with second resolution to not change existing inference results.
Wouldn't expect too much interest in trying to infer timestamps with milli- and microsecond resolution. The nanosecond resolution can already parse the lower resolution ones.