The IPSW files themselves are not signed. This is done on the server side by Apple, so as the post above says, you're stuck. Not even 9.3.5 is being signed anymore.
Apple has stopped signing IPSWs as they release new versions of iOS since iOS 3. That's June 2009. So "this time" has been the last seven years.
If you want to count from when Apple started using AP Ticket, which was iOS 6, and therefore effectively ended a jailbreak method to downgrade, then that would be September 2012. That would be four years ago.
So, Apple has stopped signing for years when done with a firmware. It's not like this is the first time they've ever "screwed it up for their users". Unless there is some sort of special significance you are attaching to a routine Apple activity we don't know about?
There was ways many years ago to stitch shsh blobs to an older firmware version and downgrade.
That method has been patched and those shsh blobs are unique to each device.
You cant use blobs from Eric's iphone and downgrade your iphone for example.
But all that is not really relevant any more. In short, no there's no way currently to downgrade.