First, it was a setting in system preferences to allow unsigned apps.

Then, that option was removed and it was the a are-you-sure-you-want-to-open-this dialog on every first run of a new or updated app.

Then the "Open" button on the dialog was removed and put in the Security preferences pane.

When System Preferences became System Settings a couple releases ago, that button got moved below the fold in the Privacy and Security prefpane. This is the current state today and is infuriating for some OSS apps, especially niche ones.

At one point, there was a command-line way to clear the xattr 'com.apple.quarantine' on the .app, but that also got nuked in a recent update.