The "green" was used for alot of years and pretty much the whole engine cooling system during that time was cast iron with copper radiators/heaters with rubber hoses and a few steel parts like water pump impellers and thermostats.
As manufacturers started using different metals, like aluminum, the chances for galvanic corrosion increased, but as long as coolant was changed every 2 years protection remained, except not every one does that, and on older engines that wasn't a big issue since metals were the same, with different metals it was a problem.
OAT was developed for longer term protection, 5 years, so problems would occur AFTER warranty period, my own opinion
, if people didn't change coolant when required.
There are all kinds of tests done as to which is better, IAT, OAT, HOAT, ect......., and with which engine models, years and displacement.
I can't say which is better scientifically on paper, but others have done that so I don't need to
On vehicle less than 10 years old I tend to go with what manufacturer spec'd for that year and engine.
After that I figure I am going to get leaks, lol, and go with my more practical plan short term protection, not figuring I will get that 5 years at this point, lol.