Details from here (there's also a video announcement too): https://www.warhammer-community.com/2017/08/09/new-matched-play-rules-in-chapter-approved-aug-9gw-homepage-post-1/
New Matched Play Rules
We are always eager to listen to the Warhammer 40,000 community and improve the game based on the feedback you provide, and one of the areas we get the most correspondence about is matched play. The joy of Chapter Approved is that, as we see certain themes emerge about rules that might not be working properly, we can take steps to address them. To this effect, Chapter Approved introduces several new matched play rules, all of which are designed to help redress certain imbalances or exploits within the rules that you tell us are leading to games that are not as fun as they could be. Here you can see one such example:
As you may recall, we have already amended the Instant Death rule in Warhammer 40,000 to prevent Flyer-only armies from dominating matched play games. This was met with a positive response, but the feedback we have received since suggests that it did not go far enough. Boots on the Ground helps to redress this – aircraft are soaring so high and so fast that they cannot hold objectives; their role is to support the warriors and vehicles on the ground as they do the bloody work of capturing and controlling territory.New Detachment Rules
Since Warhammer 40,000: Dark Imperium was launched, we have released several codexes, but there are still plenty of Factions – Orks, Drukhari, Necrons etc. – that don’t yet have a codex of their own. Chapter Approved gives these players a sneak peek of what many of these factions can expect when their codex is released (we are hard on work to get that done as quickly as we can), but one of the things we wanted to do was give everyone access to ‘objective secured’ right away:
As you can see, this is an ability that rewards you for grouping your warriors into faction-specific detachments by giving Troops units the ability to control objective markers even when outnumbered by the enemy. As you can imagine, it gives these units a real edge, and helps to ensure your line troops are not at a disadvantage compared to those factions that are lucky enough to have a codex already.New Matched Play Missions
A great mission is critical to a great game of Warhammer 40,000, and Chapter Approved includes a dozen brand new matched play missions for you to test your skills. One of the things we’ve had the most feedback about is about which player gets the first turn in these missions. Barring a sneaky Seize the Initiative roll, the current feeling is that the present method is a bit too reliable, and some players are developing armies and strategies to exploit this. We’ve taken this feedback on board and changed how you determine who gets the first turn in every one of the new matched play missions presented in Chapter Approved:
This change ensures that in these new missions, the first turn is never guaranteed; whilst you can construct an elite army to stack the odds a little in your favour, it still all comes down to a roll-off, so you’ll have to build an army and deploy it accordingly. If you prefer the original method though, worry not! These new missions do not replace the matched play missions in the Warhammer 40,000 rulebook, they supplement them, allowing you to choose the style of mission rules you enjoy playing the most.