The only ones I am familiar with are Pacsafe but I think they may be only shoulder/neck straps. You could attempt to modify a Bkackrapid with a wire insert if you really must have slash proof.
Have you considered the potential consequences of having a cut proof strap should a knife or razor wielding villain target you or your family?
One possible scenario a would be thief targets your impressive looking expensive camera, attempts to slash the strap and fails. He is already committed to a felony so if you are lucky he just threatens you to release your camera and any other valuables you may be carrying. Or even faster just slashes you a few times until camera released. Just a thought!
I understand what you are saying to some degree. However, my personal experience is that most pic pockets will not engage in a stabbing and those that will, simply DO SO without bothering to cut the strap first.
About the second there is not much you can do if they go that route from the back.
However, the attempted slashing of the cutproof strap will give me more than enough time to become aware.
Without sounding grandiose I have faced knife wielders on multiple occasions as part of my job as well as traveling, and I am still here. Further having a cutproof strap of that length gives me a weapon I can open carry, get through ATF and is as deadly as any knife ( and I am not talking about swinging the camera with it. )
In High School, I had a friend who told me "In a fight, you don't want to have anything tied to you!" Good advice.
Before I retired, there was an effort to provide lanyard I.D. for school employees...teachers, administrators, etc. The problem was, NONE of the people agreeing to the equipment saw the potential choking problem. Now, they all (I hope) have some sort of quick-release or break- away feature. IRC, all of our 2002 Olympic volunteer lanyards were of the disconnect type.
During the Cold War, couriers had briefcases chained to their wrists...till the "opposition" figured out it was much quicker to cut off a hand. That was replaced with a cable attached to the briefcase, and running up a sleeve, attached to a body belt.
My point? As we age, and are no longer young and immortal, perhaps it is best to not put ourselves in a no-win situation, regardless of our capabilities...likely much less than we think. The days of the sly, capable pickpocket are mostly gone. Now, It's "Your money or your life", or maybe both! AND the criminal, who may be of either sex, and as young as 10-12 has no qualms about leaving you bleeding ad/or dead.
So...Safety First; Like in Boxing:" Protect yourself (and yours) at all times!" Be aware: heightened Paranoia in some circumstances is a Good Thing. Listen to your Spidey Sense...If you feel a tingle, Leave the area with speed and enthusiasm....regardless of static from the Spousal Unit or Munchkins.
Simply DON'T go into questionable areas for the "local color" (It doesn't matter how good the little backstreet restaurant/bar/shop is said to be!) without a reputable guide or a group of friends, and don't flash expen$ive cameras or bags with large, gaudy straps advertising the brand, watches, or large rolls of money. Little point and shoot cameras have marvelous capabilities. Disinformation!...carry your equipment in a Diaper bag, or one of your wife's large over the shoulder bags. I know...doesn't look Cool, Pro Photographer. Works better if you have some young ones in tow. Before you go, place silver duct tape on your equipment, like it's holding it together. Don't overdo it. The worse your equipment looks, the less market value/attraction it will seem to have. Duct tape is easily removable, and the stickum is easily cleaned with rubbing alcohol.
In the late '70s, I once walked all over the an area of Chicago with my camera. This was in an area where a co worker, when he had to go there, hired a paroled murderer (Think BAD, BAD Leroy Brown!) to keep the denizens off his back. A resident I talked to later, turned pale, and said "You did WHAT?" I dressed in Levis, parka, and watch cap, which was close enough to looking like a local, and my camera was an old Pentax with a preset 50mm lens. Nothing happened. Could be I was just lucky. I understand that today, in some "enlightened" countries, ,if you react physically to an assaulter, Heaven forbid you should actually win! You may be in more trouble than the criminal.