All iPhones from iPhone 7 and later support NFC for third-party apps.
Most Android smartphones released since 2015 include NFC, so nearly all mid-range and high-end devices are compatible. Some older or budget models may not have NFC, so please check your phone’s specifications if it’s an older device.
KEEP works with every phone that has NFC. If your phone case contains magnets (MagSafe, Qi2, "with magnets") or you use a magnetic wallet or car-mount plate, remove it to reveal your messages. The magnets interfere with the NFC antenna of your phone.