It's a letter in the Telugu alphabet pronounced Jna.
The letter 'Gna' or 'Jna' is used to form words like ‘Gnanam’ which means wisdom/ knowledge. ‘Agnathavasi’ is another word, which means someone who is away from everyone, and cut off from the rest of the world.
In the iOS, Mac OS and watch OS update this would crash the app which it was sent in. You can watch unbox therapy video showing it to us. This is not a rumour and I have personally checked on some devices.
If you receive a notification that includes that heinous character, Springboard will reset. Springboard is the part of iOS that manages notifications. In some cases, and if you reboot the phone before Springboard resets, your phone will go into a boot loop and refuse to start up.
Apple will fix it soon and this isn't found in people who have the old update. Only the latest one. My suggestion if you haven't updated is don't update till apple releases a fix soon.