X-Plane never talks directly to your video card. It always talks to the video driver, a piece of software that acts as a translator between X-Plane and your video card. (That's a gross simplification - video drivers do a lot of amazing things these days.)
Your video card has certain capabilities - different cards can do different tricks. The video driver tells X-Plane what tricks the video card can do.
So there are really four possibilities for any given trick:
- Your video card cannot do that trick. The video driver will tell X-Plane "I can't do it."
- Your video card can do that trick. The video driver will tell X-plane "I can do it".
- Your video card can do the trick, but the driver is old and will tell X-Plane "I can't do it" because the driver doesn't realize what the card can do. (This can happen because the video driver you use might come with your operating system, before you bought the new video card.)
- Your video card cando the trick, but the driver has a bug. The driver will tell X-Plane "I can do it", but when X-Plane says "go and do it", the video driver will crash your whole computer. This happens when drivers are buggy and can usually be fixed by getting newer drivers.
- Some users have cards that can't do it and will see no change.
- Some users have cards that can do it, so they see the new feature.
- Some users have cards that can do it but old drivers and see no change.
- Some users have buggy drivers and see a crash for the first time.
GLSL shaders are definitely not required for X-Plane 860. If you don't have them, X-Plane will run the way it always did.
But if your drivers are buggy and crash when GLSL shaders are used, you will need a software update to get good drivers. (There is a way out of this...if you really can't get a driver fix, there are command-line options that will tell X-Plane "don't use this feature even if the card has it".)