The update is set to be released next month on February 28, so if you're using an older device you'll want to catch as many Pokemon as you can before that time. Right after that you will have to follow the instructions provided on the screen and your iOS will update to the 11.2.2 version.
Also a part of Niantic's memo is the fact that users on Apple devices older than iOS 11 won't be allowed to access their PokéCoins and other items.
According to the Pokemon GO Hub, the update removed the Dragon Tail fast move from Kyogre's move pool. "They will need to switch to a supported device to continue playing Pokemon Go". This is going to be a tragedy of epic proportions for those "biggest screen possible" players out there playing with wi-fi hotspots. No doubt, players who have invested real money into the game via microtransactions will be fairly vexed that they're being ditched for not having the latest iPhone or iPad.
Pokemon Go is already experiencing a tumultuous 2018, as many players are growing frustrated with the controversial EX raid feature.