Apple has stopped the digital signing of iOS 12.1.3 following the release of 12.1.4 last month. This means that if you wanted to downgrade to 12.1.3, you won’t be able to do it any longer. The act of stopping the signing of an iOS release effectively means that it is not available to consumers on the Apple software servers. If you are still running iOS 12.1.3, you can continue to do so (although there is a FaceTime bug that is fixed in 12.1.4 – so upgrade. Please).
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 12.1.4 and want to go back to iOS 12.1.3, 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.