Two weeks after the release of iOS 13.3.1, in which were fixed various bugs and vulnerabilities, cupertinos stopped signing iOS 13.3. This means that users who are on newer firmware versions will no longer be able to install an older version of iOS on their devices.
In General, anything unusual in the actions of Apple no. The company stops signing older versions of iOS after about two to three weeks after the release of a new build. This mechanism prevents fragmentation of the platform, but also positively affects the security of Apple’s mobile OS.
It is noteworthy that Apple has stopped signing iOS 13.3 almost immediately after the team Pwn20wnd reported about the readiness of the public beta version of the jailbreak utility unc0ver, which enabled you to hack iOS devices 13.0-13.3 on processors A12(X) A13.
As mentioned above, users are now able to update their devices to iOS or 13.3.1, which was released in last days of January, or install the beta version of iOS 13.4.