The app will continue to work, i didn’t add planned obsolescence features or something.
But you do no longer receive updates, bugfixes or any support.
Version 2020.1.0 is already out so if anything breaks with Nextcloud 16.0.8 in two weeks, it won’t get patched.
If you want to update later, make sure to update to Passwords 2019.12.1 first, then Nextcloud 17, then the latest version of the app and then the latest version of Nextcloud.
The new browser extension which will be released this year will cause issues.