Pairing an old Android charger with a new USB-C iPhone generally works at a basic level as long as the physical connector matches — the open USB-PD negotiation process means the two sides settle on whatever power level both support, defaulting to a safe minimum if there’s no overlap in fast-charging standards.
Bottom line
Expect it to work at some charging speed; whether that speed is fast depends on whether both ends share a common fast-charging standard, not just a compatible connector shape.