It is all free software under the GPLv3, with the source available.
Needs Android 8 or later. No Google service required.
Get the APK from GitHub F-Droid, coming
Android asks you to allow installation from whatever you downloaded with and Play Protect warns that the developer is unknown. Both are normal outside the stores: tap More details, then Install anyway.
If you use the SMS channel, that permission may come up blocked: unblock it from the app settings, in the top-right menu, under the entry about restricted settings.
Optional. Copy the files to a host with PHP 8, make a folder writable and fill in three keys in the settings.php file. A check page tells you what is missing.
The package contains settings-default.php: rename it to settings.php and fill in the keys. The file holding your keys then stays out of the package.
Download the new APK and open it over the old one: data and settings stay put. Do not uninstall first unless you want to start over.
Just copy the new files from GitHub over the old ones. The settings.php file is not part of the package, so your keys stay where they are and no update can touch them.
If a version adds settings, you will find them in settings-default.php and can carry them over by hand: anything missing keeps its default.