Apple decided to drop the iOS 9.3.4 update in order to patch the IOMobileFrameBuffer Exploit used by Pangu in their Jailbreak utility they released for x64 devices for iOS 9.3.3, to iOS 9.2.
From a first glance at the Apple Security Support page, we can see that iOS 9.3.4 does nothing more than patching Pangu's Jailbreak.
This patch affects both the computer and the no computer Jailbreak version and if you want to keep your Jailbreak YOU MUST NOT update to iOS 9.3.4. If you already updated to iOS 9.3.4, there is a short window of time in which Apple will still sign the iOS 9.3.3 and you can downgrade back using the appropriate IPSW file.
Keep in mind that there will be NO OTHER 9.x Jailbreak. Pangu's is already focusing on iOS 10 Jailbreak (iOS 10 will be released in September this year), and TAIG, well, last time TAIG dropped a Jailbreak Utility, the version was iOS 8.4, which does not give very much hope on iOS 9.3.4 Jailbreak.
To sum up, do not update to iOS 9.3.4 unless you want to be locked out of Jailbreak until September.
x32 Devices do get this update as well, even tho, there have been no Jailbreak utilities for them. You should still not install the update.
==Something to read==
Jailbreak Playlist: //www.youtube.com/playlist?list=PLn7ITOG6DYVRmbAXR6yLlFl0fVngKospy