Is it possible to remove a screw that has an allen key slot, if the screw slot has worn away notches and the allen key can't turn the screw anymore as it can't grip it.

Possible? Yes. Easy? Don't count on it. Start with a shot of spray lube. I prefer a silicone oil to WD-40. If it sticks out, hammer on it. If it is recessed, use a flat tipped punch. This will help loosen it and may deform the head enough that the Allen Wrench will then turn it. If not, grind a taper on the next larger size and try hammering it into the recess. You can try using a sharp punch sideways on the head to turn it. As a last resort, drill into it and try a screw extractor.

The old coot I once worked for called the acetylene torch the heat wrench. Heat it red hot and quench it. He also maintained there was no such thing as an easy out. He bad mouthed people that brought things into the shop with a broken off Easy Out in them.

I like a penetrant like PB Blaster. WD is good.
Easy Outs work, They are hard and brittle. Hard is good, cut into stripped screw.
Brittle is bad. Will break if side force applied. Turning is key.
A "T" handle would be better than not. If it works, you will thank me. If it breaks,
You will curse me and the tool. Like above, Heat expands.

Are you sure that it is an Allen head screw, since you used the word slot, An Allen head has a hex in the head. Unless the head is recessed in a counterbored hole, see if you can get a bite on the outside of the head with a pair of vise grips. If you don't want the heat of a torch that might cause harm to the rest of the asembly, you might consider tack welding the Allen wrench into the bolt to keep it from spinning in the worn out hex.

May not even be for a allen key, many screws now have special slots for specially designed tools. Torx,hex,phillips,torx&ball, just to name a few and don't forget it may be metric or standard in all types. Key here is to know what kind of tool is needed for what slot. I bet it is just a metric size