If you manage to install a standard Netflix IPA (extracted from the App Store), it runs exactly as expected. However, most users seeking IPAs are looking for "tweaked" versions (e.g., DVDNetflix or similar unauthorized mods).
iOS 15 is currently supported by the iPhone 6s through iPhone 13. However, as time passes, Netflix may drop support for older iOS versions. netflix ipa ios 15
If you want Netflix on iOS 15 without sideloading headaches: If you manage to install a standard Netflix
This is the most common legitimate method for installing IPAs on iOS 15 without a jailbreak. iOS 15 is currently supported by the iPhone