"Changing rules is a different animal than giving individual cards errata." I agree but isn't making Planeswalkers legendary a functional errata too though? They now have interactions with cards like Yomiji Who Bars the Way, Honor-Worn Shaku, and Empress Galina where none existed before. If that's not a functional errata, what's the distinction?

To play Magic you have to know the rules. The cards only explain when they break the rules. So if a rule changes, we expect players to learn how the rules have changed. But for individual cards, we try to make them do what they say as best they can. If a card has lifelink, for example, it does whatever the rules say lifelink does even if that rule changes over time (as it has). The rules for the legendary supertype has changed. Players have to learn this change and apply it to all cards in the game. Changing the Nephilim is not changing an overall rule. It’s changing how five specific cards work which would require learning five exceptions. That’s the difference.

At long last, it is my birthday. :) Could I get some trivia on one of my favorite cards, Teferi's Puzzle Box, or my commander, Empress Galina?

Sometimes we like to make cards that radically change how the game works to allow players the ability to either build their deck around it or just have a very untraditional game. Teferi’s Puzzle Box was one such card. There was a lot of debate around it but we chose to keep it in the set. Happy Birthday!