Change in "can't be countered" wording
The reason for this change was to "clarify that a card's ability is used at the time you cast it", according to Aaron Forsythe.

This change was not only limited to "can't be countered", but to all abilities that matter when the card was being cast (when it is a spell). See Talara&#39;s Battalion, which went from "Cast Talara's Battalion only if you've cast another green spell this turn." to "Cast this spell only if you've cast another green spell this turn."

You can read about it here. 

I can't give a definitive answer about Nezahal, Primal Tide. But we know from past history that sometimes in large Oracle updates, certain cards have been missed, and they get updated in future updates. You can find an example of that in this article, where it explains that Decree of Annihilation should have been updated previously, but was missed and was updated later.