The biggest change is good news for Urza's Saga fans and bad news for Blood Moon fans
Rules Update: Sagas
Speaking of Sagas, we're making a small but impactful rules update with this release. With the advent of Saga creatures, it's much easier to get into a situation where a Saga on the battlefield loses all of its abilities but is still a Saga enchantment. The result of this interaction was previously unintuitive to many players: since that Saga had no chapter abilities, the game defined the "greatest number among its chapter abilities" as zero, and no matter how many lore counters that Saga had on it, it would be sacrificed as a state-based action once it wasn't the source of a chapter ability on the stack (usually right away).
Starting with the release of Magic: The Gathering—FINAL FANTASY, we're updating the rules: If a Saga has no chapter abilities, it won't be subject to the state-based action that would cause it to be sacrificed due to how many lore counters it has. Similarly, it won't be subject to the turn-based action that adds a lore counter to each Saga you control at the beginning of your first main phase each turn. For example, if your Summon: Bahamut with one lore counter on it loses all abilities because someone attaches Observed Stasis to it, it'll won't gain lore counters via the turn-based action until it has chapter abilities again (probably because you removed the Observed Stasis somehow.)