The application are the bolts that hold car seat slider rails to the floor of the car and to the underside of the seat. Presumably, it’s important that the seat stay put in a wreck.
The bolts that came out were 8.8 - not 100% sure if they’re original, but appear to be. Some have suggested that in a wreck the seat rails could be subject to shear and a bolt that deforms may be better than one that’s brittle and breaks. Others have suggested that higher strength bolts should be used.
It all depends on the direction of impact whether the bolts will be in shear or tension. A frontal would be tension, a side would be shear, HOWEVER it's not quite that simple. I've made a few custom seat attachments for various hot rods and I worked with automobile seating in another life. The seat belt attachments take the brunt of the tension loads ..... IF they are being used. The seat itself, probably weighs around 75 lbs or so, the human occupant weighs more than that.
If an unoccupied seat comes out of the floor pan in a frontal, it would have to be VERY rusty. The backing plates on seat belt attachment bolts are supposed to survive the collision, rust or not.
You can go to the dealer and get OEM bolts OR have them read the specs for you OR look them up online. Grade 8 SAE is what I've used in my truck and I try to use the OEM bolt holes.
Seat belts work because they restrain the occupant from careening into the instrument panel and or through the front windshield. I've seen actual accidents first hand where the unbelted occupant almost went through the windshield, but for the plastic laminate, she would have. I myself, was in an unbelted frontal collision, my fault and not paying attention to the vehicle in front. My head cracked the glass and when I braced for it, I wrapped the steering wheel almost around to touching the steering column. I was very fortunate. The car in front was dead stopped and turning left, a '59 cat eyed Chevy.
So much for that.
Also remember there are 4 bolts, not just one, and for the seat to come loose all four would have to fail.