LineageOS

LineageOS is the custom ROM with the widest reach, officially supporting well over a hundred current and long-abandoned phones and keeping them patched and de-bloated. Version 23 tracks Android 16 and supported devices get monthly security updates. It ships close to stock Android with no microG or Google services by default so it’s clean but not especially hardened, verified boot is inconsistent and security depends on the per-device maintainer. It’s the best way to extend the life of an old phone, not the pick for a high threat model. Add microG yourself if you need app compatibility.