And yet "Hyrum's Law" famously says people will come to rely on features of your system anyway, even if they are undocumented. So I'm not convinced this is really customer-centric, it's more AWS being able to say: hey sorry this change broke things for you, but you were relying on an internal detail. I do think there is a better option here where there are important details that are published but with a "this is subject to change at any time" warning slapped on them. Otherwise, like OP says, customers just have to figure it all out on their own.

Sure, but the court isn’t going to consider hyrum’s law in a tort claim, but might consider AWS documentation - even with a disclaimer - with more weight.

Rely on undocumented behavior at your own risk.