What does it mean if IPSW file is not signed anymore?

Easily spoken, you could say ipsw files are not complete. They're missing that so-called signature. It's like a contract; it has no value without the signature. Every time you install new firmware on the iPhone, iTunes calls to the Apple servers and ask for that approval signature. For numerous reasons, Apple doesn't give out those signatures for older firmware versions.

There's not much you can do. The only thing you can do is downgrading to 8.4.1 and hope one day somebody releases the jailbreak for it. But for now, you're stucked.

UPDATE: As of Oct 2015 Apple stopped signing 8.x. As of Oct 2016 Apple stopped signing 9.x. While this answer is not correct any more it still describes the general idea behind Apple firmware signing.

You can obtain a signature which developer ID, and Date Signed. if you know what the orignal checkSum is it is easily modified using notepad and fiddler 4. For file signatures, push a Untampered IOS Update through 3utools, Flashing with proFlash. Before starting the process, have fiddler 4 open and then start the flashing. during the process fiddler will recieve a response for the ios update md5 checksum from its servers, and the file signature files later on. Once complet to find the signature go into the 3uTools system files, and inside the Other folder you will find a zip file call ss_ios.zip . Inside that zip you will find a .sig file. that is the file you must use to sign your modded files. Also If the Last Modified Date doesnt match the original file, you will also fail. Though changing the metadata on the last modified date is easy.

Another note if the crc32 value doesn't match, the buildManifest or the device tree information for that file. you will also fail.

Apple uses md5 as an initial check, then crc32, date modified, File Signature. If any fail the image is reversed from disk, and the old image is restored to last state.

Hope this help you out!!

USE 3uTools to push the ios. in Itunes if your md5 checksum value isnt write then it says ipsw not signed. FYI

The above answers are right...that if the ipsw is signed you can install (flash) it through iTunes (PC/MAC) and the unsigned ipsw can't be install with iTunes

But if you are a Obstinate Person and still want to downgrade your ios version....then download the wished ipsw firmware from here--> http://ipsw.me and install (flash) with 3u tool....download from here--> http://www.3u.com