But at least if it's available as a slot then a module is *able* to use it with limited ABI going backwards. A new function doesn't allow that.

And yeah, it means they have to write more complex code than just a static array definition, but people are willing and able to do it.

I don't think there's anything about the current FromSpec that suggests all the parameters have to be interpreter-agnostic. And the design does suggest that we'll add more later without having to break the ABI (otherwise it would be a regular struct).