The strength of a pool slide depends on what it has been constructed from. Materials like fiberglass or acrylic were quite popular back in the day. Now, most slides are made from molded polyethylene.
Polyethylene is basically very heavy-duty plastic. It is capable of withstanding the harsh rays of the sun, has some flex to avoid cracking, and is largely a very durable product. Maintained properly, most pool slides are usually strong enough to last many swimming seasons.