The 5W power adapters that came with the iPhone 3G are still sold in the US.
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For now
The 5W adapter was included all the way to the iPhone11 pro. Rare charging was replaced with 18 W adapters with USB-C support. With the release of the iPhone 12, Apple stopped packing its smartphones with chargers altogether.
It is noted that using the old adapter, you can try to charge newer iPhones, but charging will take at least 3 hours.