After quite a few teasers and previews, MTG Arena has now officially added the Ravnica Allegiance expansion alongside a massive update. You can expect to see 259 new cards to collect (111 commons, 80 uncommons, 53 rares and 15 mythic rares), a duplicate card protection system for packs, somewhat reduced rewards in Constructed Events, as well as significantly cheaper side-events.
Since the cards themselves are the most important part of this update, you should head on over to the official website in order to learn just what Ravnica Allegiance has brought with it. My personal favorite out of the bunch is Electrodominance, which besides having the perfect Izzet flavor text, also enables some truly outrageous and flashy combos - and I certainly can't say no to those!
I also hope you're a fan of red as Judith appears to be everywhere currently
When it comes to the duplicate card protection system, it's about as simple as it gets. If you would open a pack and receive a fifth copy of rare or mythic card, you will instead be granted a random card of equivalent rarity from that set. On the other hand, if you've completed the set, or if you receive a fifth duplicate rare or mythic outside of opening card packs, you will be granted some gems in order to help soften the blow. Uncommon and common duplicates will still add a very small percentage to your next vault opening, but since they're never really the bottleneck in creating decks, this shouldn't be too much of an issue beyond your first few weeks.
As a consequence of these amazing changes, the developers have now also nerfed the payouts for Constructed Events. You can find all of the details by checking out the latest developer post, but long story short, the average player will be getting noticeably smaller amounts of rare and mythic cards by playing in the Constructed Event. A sizable nerf to be sure, and one that I don't exactly agree with, but I suppose some sacrifices had to be made in order for duplicate card protection to become a thing.
That's pretty much all of the major changes, but if you're looking for some more information, you will find the official overview over at the MTGA forums. Have fun with all of the new cards, and don't forget to use the "PlayAllegiance" code to claim your 3 free packs!