With the release of iOS 12.1.3 a couple of weeks ago, Apple has stopped the signing of iOS 12.1.1 and 12.1.2. Effectively this means that downgrades to those previous versions are no longer available after you have upgraded to .3 on your iPhone, iPad or iPod touch.
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 consumers, what this means is that if you upgrade to iOS 12.1.3 and want to go back to a previous version, you are not able to do so. The older versions will not be served up to you from Apple. For most, this is not a big deal.
Don’t be surprised if you see this process repeated very shortly. Apple is expected to release iOS 12.1.4 this week to fix the Group FaceTime eavesdropping bug. That means by the end of February, 12.1.3 will likely no longer be signed as the iOS upgrade train chugs along.