well I only find out these things from groups that I belong to and various information i receive from people that are personal trainers. The part that makes the most sense to me is that it is a very unnatural movement that there aren't very many situations that i can imagine that a movement that a bench dip does would ever occur naturally. Chin ups or pullups for example involve a movement that throughout the evolution of humanity would have happened all the time.
I don't pretend to be the origin of any of these ideas, i just know that i've seen people that did the "wrong" exercises and appeared to be in fantastic shape for many years... now they are older and they are paying for these mistakes that they didn't even realize were mistakes because we didn't have the technology at the time to determine that they were in fact, mistakes. If there are other options to work the same muscle groups I don't see any particular reason to continue doing one that there is evidence it causes chronic pain later in life.
There is so much advice online and some is contradictory, so it can be hard to know what is good or bad. Just have to pick who you trust. I'll be considering which exercises feel unnatural in future. Cheers.