Using it in a product without a copyright notice of OSS is a violation of the license.
Please trace back to the source to identify the copyright holder and confirm the contents of the copyright notice. However, there may be cases where the author does not want the name of the author to be written, so please respect the author's wishes in that case.
If it was obtained through multiple companies, it may have been altered along the way and the copyright holder may have increased.
The same action should be taken when only the name of the license and a link to the license document template are included.