Struts

Yes. You generally buy two new shoes or socks at the same time. Matched struts provide balanced ride quality and equal response to bumps in the road. The only time you would replace one strut is when one is damaged in a crash and they were both relatively new. Same applies to the very rare failure of one strut when the car is still in warranty. When they are that new, a single new strut from the manufacturer will still match the other strut.

At the mileage you listed it is about time to expect to find worn struts that are not damping the road bumps for best comfort and handling. By the time the car gets to be around eight to ten years old, a conscientious mechanic will recommend new struts that already have new coil springs installed. That will restore the suspension ride height which is critical for best tire wear along with maintaining proper alignment.

Struts usually are not covered under warranty anyway unless there is severe leakage or a mechanical problem like the mounting bracket broke off, (very rare). They are considered wear items and are expected to need to be replaced periodically. Same is true for brake linings. Those are almost never covered because the manufacturer has no way of controlling abuse, and they're expected to wear out. Warranty only covers defects, and parts that are expected to wear out are not considered defective. Your case might be different, but at 100,000 miles, any car is ready for new struts. Chalk that up to cost of ownership.