Wizard's somewhat recently changed the wording of uncounterable spells to be:
"This spell can’t be countered."
The old wording was:
"[Card name] can't be countered."
This change seems to have happened as part of the April 2018 rules change where spells and abilities with invalid targets changed from being countered by game rules to just fizzling. As a result of this, they needed to change the wording on cards that said "[Card name] can't be countered by spells or abilities." But this doesn't explain the change from "[Card name]" to "This spell", nor does it explain why they needed to issue errata for cards that didn't have the "by spells or abilities" component.
So why did they make this change?
Also, the Gatherer Oracle for Nezahal, Primal Tide appears not to have been changed. Is this an error?