I'd probably leave out this sentence. It is extraneous and doesn't really explain why this is a "bad thing" (it can also confuse some contributors because they'll notice that they can do exactly the same thing with private methods and attributes).

I'm not particularly in love with the wording here, but I do want to stress that point extremely strongly: if an attribute is public, the implementation should be robust against the end user modifying it; otherwise, make it private, or hide it behind a getter or a property.
If you have a better wording I'm all ears :)

Or add a "And therefore once we have released that API the behavior, arguments, and names can not be changed without going through a multi-version deprecation process." ?