From version 1.5, the macOS application will now be notarised. It means that when you install it you will no longer see a warning telling you that the app hasn't been checked for malware. You also won't need to go in the OS security settings to launch the app.
Notarisation has been required since macOS Catalina however it took a while to implement it as the process is complex and error prone.
First the app has to be built and uploaded to Apple's servers. They then automatically check the app for any malware, a process that can take 5 to 15 minutes (and presumably more if the app is incorrectly flagged as malware). Once it's done, Apple creates a notarisation ticket for the app.
Then whenever the app is started, Apple check on their server if the app has a notarisation ticket (a process that recently caused "apps taking minutes to launch, stuttering and non-responsiveness throughout macOS"). It's also possible to staple this notarisation ticket to the app, so that it's possible to check the ticket even when the computer is offline.
All that has now been automated in the Joplin build pipeline, and will apply to all macOS app future versions.