"The behaviour of X is undefined" is a well recognized phrase to indicate an action is unsafe. Common usage trumps logical analysis in practice (assuming you buy the logical analysis in the first place). "You must not do X" is an alternative (commonly used in internet standards).

I've also encountered "The behaviour is subject to change." (which is rather meaningless since everything is subject to change) and "One should not rely on the current behaviour.", but those have a different, weaker meaning. They imply the action is acceptable if a suitable upgrade strategy is in place.