As expect, Apple has stopped signing iOS 12.1.4 following the release of iOS 12.2. This effectively means that downgrading back to 12.1.4 is now not possible. The act of stopping the signing of an iOS release effectively means that it is not available to consumers on the Apple software servers.
Apple routinely stops the signing (the making available on their servers) previous versions of iOS when a new version is released. This usually happens two or three weeks after a release, giving the Cupertino company a window to pull it back should there be a major issue found after it is released to the public. Doing this also helps Apple keep the number of supported versions under control for their support teams.
For most end users, a previous version being removed is not a big deal and only really impacts those that “jailbreak” their devices to run custom builds of iOS on their iPhone or iPad.
The bottom line readers is that if you have upgraded to iOS 12.2, you can’t go back. If you haven’t upgraded and still are on iOS 12.1.4, you can stay there for now as you won’t be forced to upgrade for now. Given the security and performance improvements in 12.2, along with Apple News+ support, it is probably a good idea to upgrade at this point.