I can honestly say that I thought that this was true early in my career, think 1980’s
We now know better.
If you do not currently have back pain, arching your back during bench press is most effective and safe for your shoulders. I am not a fan of the very large arch that some powerlifters use. I prefer a small arch in the back to improve shoulder stability and leg drive
If you currently have back pain, it depends if extension causes you more pain. If bending backwards increases your low back pain, try to bench press with back flat.
If you have pain with arched and with flat back: reduce the weight so you can do it without pain increase
Do you have pain?