Update: Apple no longer sings older iOS versions and everything is back to normal. Do not downgrade.
If you lost your jailbreak or if you are eagerly waiting for a jailbreak, you most probably do not have to wait any longer. It seems like Apple has made a mistake and is suddenly signing a bunch of old iOS versions, all the way from iOS 7 to the newer iOS 10, for a bunch of different devices that support this firmware.
If you want to downgrade, now is the chance as this may be a miracle which happens once upon a lifetime. Nothing like this has ever occurred before and the even has the jailbreak community in shock. Social media is exploding as Discord and Reddit try to grasp their userbases together and figure out what happened.
It's hard to point out which iOS versions are currently begin signed, but odds are that if your device supports a specific firmware, it can be downgraded. Activation issues have shown up for older versions like iOS 7, however, versions like iOS 10.1.1 or 10.2 are signed and available for people to downgrade to. These versions support Cydia Installer and Cydia Substrate fully, which means the jailbreak is not lacking in any way (other than stability in a few specific cases).
If you're waiting for an iOS 11 jailbreak you might not want to downgrade, however, make sure to save your SHSH Blobs for the future in case something goes wrong. You can do that by visiting the TSS Saver website and following the instructions.
In order to download the correct .IPSW for your device and check signing status, you can visit IPSW.me, which is the to-go website for these use cases.
This is indeed an iconic day for jailbreaking and will go down in history as "the Apple Singing fuckup", unless Apple does not revoke the signing status for these versions, which is unlikely but possible as pointed out by some people.
We do not know the reason behind this, but what we know is that this may not last forever so make use of the situation while you can.
What are your thoughts? Are you as overwhelmed by this as we are? Let us know in the comments.