It would indeed be a breaking a change. Since CommonMark aims to be highly compatible, it is unlikely that this option would be adopted.
Interestingly, in Markua _ represents emphasis but renders _my emphasised text_ as my emphasised text by default, with the type of emphasis configurable as a global setting. You can write something like this to make it explicit and toggle the italicize-underlines options on and off:
{
italicize-underlines: true
}
# Chapter One
stuff
foo _bar_ baz
lorem *ipsum* dolor
stuff
This is another language though, one which doesn’t have the compatibility constraints of CommonMark. If CommonMark were to override the _ syntax it would need to be turned off by default, creating divergence between documents that render emphasis one way and documents that render it another. This would create confusion for writers switching between different CommonMark documents. You could have an associated override specification for defining what class additions should map to what syntax variations, but that’s starting to get messy.
I do think there is a legitimate use case for presenting text as underlined, for example by rendering emphasis as <em class="underline">. As a global setting, this can be done via a preprocessor. For particular variations, you could use the proposed class syntax extension, e.g.
*my text*{.underline}
The question is whether we need a specific syntax for this. Perhaps the class syntax is sufficent?
