I love free software, and I'm doing this because it's fun. I have no bad intentions, and I will never monetize or sell your information, and this service and software will always stay free and open.
Neither the server nor the app record any personal information, or share any of the messages and topics with any outside service. All data is exclusively used to make the service function properly. The only external service I use is Firebase Cloud Messaging (FCM) service, which is required to provide instant Android notifications (see FAQ for details). To avoid FCM altogether, download the F-Droid version.
For debugging purposes, the ntfy server may temporarily log request paths, remote IP addresses or even topics or messages, though typically this is turned off.