Yes, it will work.
All higher-level Bluetooth devices have backward compatibility but the reverse is not true.
The iPhone 6 does not support Bluetooth 5.3 that is a known fact. The iPhone 6 model is equipped with Bluetooth 4.0 technology. While Bluetooth 4.0 is capable of pairing with various Bluetooth devices, it has a lower data transfer rate and may not offer all the features and capabilities of Bluetooth 5.3.
When considering new headphones with Bluetooth 5.3, it's important to check their compatibility with Bluetooth 4.0 devices. Many Bluetooth headphones are backward compatible, meaning they can connect to older Bluetooth versions, including Bluetooth 4.0. However, certain advanced features of Bluetooth 5.3 may not be available or fully functional when connected to a Bluetooth 4.0 device.
Before purchasing new headphones, it's advisable to review the specifications and compatibility information provided by the manufacturer. Look for Bluetooth compatibility with devices running Bluetooth 4.0 or higher to ensure a successful pairing and optimal functionality with your iPhone 6.