The first victims of the Pioneer format's aggressive approach to bans have fallen, and it's hard to feel much sympathy for the cards now riding the pine.
We all knew Felidar Guardian was not long for the Pioneer format, and the fact that it got any time to do its thing might be more surprising than its banning. There is a reasonable argument to be made that the constraints that Cat Combo placed on the format could briefly function as net positives. Creature removal and early interaction form a good starting point for exciting gameplay and meaningful decisions.
However, like the Splinter Twin analog in Modern, the combo is just too ruthlessly efficient at what it does. Punishing every other deck when the format gets too wide to include adequate sideboard and maindeck answers sets artificial limitations on the potential diversity of the format. Felidar Guardian decks were only going to get better as the format went on, and as their other angles of attack figured the correct axis, playing against the deck would quickly become a frustrating experience. In summation, good riddance.
When it comes to Leyline of Abundance and Oath of Nissa, it feels like these bans were judgement calls. Leyline of Abundance is likely dying for Nykthos's sins. There is a return to Theros right around the corner. Maybe there is a good amount of interesting gameplay to squeeze out of Nykthos that doesn't feel quite as broken as Mono-Green Devotion's absurd "fifteen mana on Turn 3" starts.
Oath of Nissa strikes me as a weaker card than Once Upon a Time by a good margin. Banning Oath does slow down a Kethis deck that looks like it is starting to get tuned, but the goals of an Oath ban feel a bit more nebulous to me. I don't think anyone would argue that green isn't in need of some nerfs, though. Why let them easily cast all the busted three-mana planeswalkers? Let's at least let them miss a secondary or tertiary color occasionally.
Ultimately, we're splitting hairs here. The right decks ate nerfs, and now we are granted with the greatest gift you can bestow upon a Magic player—a new format to figure out. Tom Ross will go deeper on some decks that are completely unlocked by these bans, but I want to take a more measured approach and identify what the impact of these bans are on the decks already sitting near the top tiers of Pioneer. Let's start with the biggest losers.