Downgrading the operating system version of any iPhone, iPod touch or iPad will render the device's data wiped. Your iOS 11.2.1 data is not an exception if ever you will revert to the version that is potentially able to get jailbroken.
Apple is signing iOS 11.1.2 at the moment and this would be cut off anytime soon. If you're keen on jailbreaking your device in iOS 11.1.2, you better downgrade as soon as possible.
How to downgrade to iOS 11.1.2
Here's a step-by-step guide on how you can downgrade from iOS 11.2.1 to iOS 11.1.2 without deleting all data from your iPhone, iPod touch or iPad:
Step 1: Get the iOS 11.1.2 IPSW file for your device. Links are provided below!
- iPhone 7 (iPhone9,1)
- iPhone 7 Plus (iPhone9,2)
- iPhone 7 (iPhone9,3)
- iPhone 7 Plus (iPhone9,4)
- iPhone 6s, iPhone 6
- iPhone 6s Plus, iPhone 6 Plus
- iPhone SE, iPhone 5s GSM, iPhone 5s CDMA
- iPad Pro (10.5-inch)
- iPad Pro (12.9-inch) (1st & 2nd generation)
- iPad (9.7-inch)
- iPad Air 2, iPad mini 4, iPad mini 3
- iPad Pro (9.7‑inch)
- iPad Air 1, iPad mini 2
- iPod touch (6th generation)
- iPhone10,4 (iPhone 8)
- iPhone10,5 (iPhone 8 Plus)
- iPhone X
Step 2: Enable iCloud backup for Health. To do this, go to Settings > iCloud > iCloud Backup > Back Up Now.
Step 3: Connect your iPhone, iPod touch or iPad to PCX or Mac using a USB cable. Make sure your iOS device has the latest iTunes.
Step 4: Open iTunes on your iOS device and see the device information of your connected device. Manually restore to a chosen IPSW file. For Windows PC users, hold down the Shift key and click on the Update button. For Mac users, hold the Option button and click on Update.
Step 5: Choose the downloaded IPSW file when prompted. This will be saved on your computer. iTunes will restore the device to iOs 11.1.2.
Step 6: Once finished, delete your existing iCloud backup and then create a new one from your iOS device.
Step 7: Wait for the backup to complete before repeating the process from Step 3 to Step 4. Note that you will do Shift + Restore (Windows) or Option + Restore (Mac) using iTunes.
Step 8: This will completely restore your device to iOS 11.1.2. Set up the device by selecting the option to restore from iOS 11.1.2 iCloud backup that you made in Step 5.
Your iPhone, iPod touch or iPad is now ready for iOS 11.1.2 jailbreak. You just have to wait until it is available.