There is a danger in implementing it as a once and I believe this is why it is an attribute. The issue is that if someone wants INTERNAL to have pre_ecma_mapping and the other one no, they always need to disable or enable that setting before using a feature of INTERNAL.

I'm not sure what is best. I think having a once object of INTERNAL might be better because at least you can have 2, one with pre_ecma_mapping and the other without and you can easily choose which one to use depending on your context.

What is indeed missing is the ability to recover old storables that might have an instance of INTERNAL. If it was the case, would the added attribute matter?