Stuck pedal, I've tried everything I'm looking for suggestions, when basically everything has failed to remove a pedal. It's a pretty new bike (Trek 520). The drive side pedal came off fine, the non-drive side did not. I am aware that it has left-hand threads.
I initially used a 15mm pedal wrench, and tried with an allen key on the other side as well. Now, the flats on the bolt are completely stripped.

I used a metal file, to file new flats onto it. Then, I placed it on its side, and put a little WD40 into the bolt, so it could soak for about 30 minutes.

Then, I used a butane torch on the spindle, where it screws in.

After all this, I put an adjustable clamping wrench to it. The bike is upside down.

It stripped the teeth on the clamping wrench, without budging.

This is absurd. The only thing I haven't tried is a bolt extractor. After all of this, I don't have much faith in it.
It seems like I'm going to have to replace the entire crank. The one that comes with it is:
Shimano Alivio T4060, 48/36/26, 2-piece w/chain guard, 175mm length
However, due to high demand, and limited resources from the pandemic, this has been sold out everywhere for some time. :/
Edit: Update. First of all, thank you, the responses here have been awesome, this is a terrific community.
I didn't have access to a vice, so I ended up taking the bike to my LBS. We removed the crank arm, stripped the pedal down to the spindle, and put it into a vice there. Flash forward to three of us working it, two pushing off the wall, and using a foot to torque the arm, while another alternates a blowtorch, and spraying it down with penetrating oil to the point of slightly deforming the arm. Whatever black magic is holding this thing, it will not let go.
The Alivio crankset isn't set to be back in warehouses until June, so we ordered a 175mm, left Sora arm for $30.
I think your pedal is a lost cause.  Even if you did get it off in the current condition, reinstalling and later removal will make later-you hate current-you.
I would disassemble the pedal from the outside - remove the axle cap, locknut, cone, and bearings, then slide the pedal cage outboard.  You may have a second set of bearings inboard, or a bushing.  The bearings are probably separate, but might be caged.  These are all &quot;spare parts&quot; now.
Now you should have a bare metal shaft poking out the side of your bike crankarm.  It will be hardened metal, and probably chromed and oily.   I would use a grinder or a file to create two flat(ish) spots on either side of the widest part of your axle.  Don't need to be perfect, need to provide a place for grip.
Then I would use either a pair of stilsons (aka a pipe wrench or monkey wrench) or I'd put the bike horizontally with that axle pointing down into a secured bench vise.  At that point you should be able to use excessive leverage to back out the axle.
Think three times about which way you need to turn the axle.  It should be left-hand thread but that flips if you're working from the back-side.  Its easy to be tightening when you want to be loosening.

Once its out, examine both the internal and external threads.  Identify why it was so wedged, and whether there is damage inside the crank.  If you have a 9/16&quot; 20tpi left hand tap then chasing the thread gently would be a good idea.  If the thread is munched, then consider options.
Finally, go shopping for new pedals.  Whether you want to stay with flats or go to clipless/cleats etc.
When you do install replacement pedals, use something as an assembly lubricant.  Grease at a minimum and is better than nothing.