They did not go anywhere, they are just not a prominent today as they once were.
The toning shoes (or more likely called the unstable footwear today) were that category of footwear that made extraordinary promises that were not and have not been supported by the evidence. The shoes tend to have a rocker under the sole that makes them unstable. Because of this instability, the muscles work harder and the gait is different. It was claimed that this could cure cellulite (it can’t) and increases your exercise to give you a tone up (again unsubstantiated by the actual evidence). All this resulted in litigation and fines from regulatory agencies for false advertising.
All that did not mean that the category of footwear did not have some use. The very nature of toning shoes is that they have some sort of rocker on the sole which can be quite useful for people with osteoarthritis in the first metatarsophalangeal joint. The change is gait reportedly help some people with postural and low back problems, and can also make worse some people, so some trial and error was needed to see if they can help. More research needs to be done to see who they can best help and what the indications may be.
Do not write the toning shoes off just because of the litigation and embarrassing celebratory endorsements they used to have. They do have their uses.