The glass door of my oven toaster and grill has broken. Can I use a thick glass (not toughened/tempered) in its place?
You can't use just any sheet of thick glass for your oven door. Glass for your oven has to be tempered to resist high temperatures. Untempered glass will likely crack pretty quickly with regular use. Moreover, an oven door usually made of glass which breaks into "rough pebbles" rather than sharp shards if it breaks, to decrease the risk of injuries if the glass does crack. If you replace it with a regular thick glass pane you run the risk of (worst case scenario) very sharp, very hot shards of glass exploding all over your kitchen one day.
Like @M.K. stated in the comments, your best bet is to contact the company and ask about replacements, or a reputable company that repairs appliances. They can easily secure an appropriate piece of tempered glass to replace the broken one.
I'm going to say, "yes", there is a reason that oven glass needs to be able to stand up to the heat. Even for a toaster oven. Safety. What Kind of Glass is Used in Oven Doors
The online order/quote for tempered glass I pulled up, guessing a 8"x12" sheet, quoted at $60 (plus taxes/shipping). Then you have to take into consideration: the glass for your specific toaster oven might have (pre-drilled) holes for a handle to be screwed on, or adjustments for the bolts that fit it so that it swings open. Perhaps you could get a replacement part by contacting the manufacturer (if the appliance isn't too old).
But it might not be worth your while to go through all that trouble. Check local discount shops (or even garage sales). I bet you could replace or upgrade the appliance for less money and aggrivation that fixing it would cause.
If you use glass at all, it should be indeed special glass made to withstand the high temperatures.
However, there is no reason to use a glass door for your oven. If you need a cheap repair, you can make a door out of any other material that is sufficiently food safe and can withstand the temperature. A sheet of steel will work, and can be shaped as needed.
This assumes that you really have a pure toaster oven. If your oven has a microwave function, do not try to replace the door with random materials, send it in for servicing.