In 2012, with OS X Mountain Lion, Apple added a feature called Gatekeeper . It had been available earlier as a command line utility, but this was the first time they made it accessible through System Preferences. Gatekeeper allowed users to control which apps could be installed on their Macs by offering three options: allow apps from App Store, App Store and identified developers or Anywhere. This was the start of Apple trying to lock Macs down, similarly to how iOS devices are locked down, but it allowed power users to install any apps they wanted.
In macOS Sierra (2019), the Anywhere option was removed. It was still possible to open any app by right-clicking and selecting Open. You had to get past a couple of warning dialogs, but it worked. Now, in macOS Sequoia, even that has gone. So how can you open an app that isn’t signed by an identified developer?
TL;DR: You can still run unsigned apps, but it’s a bit more difficult. After trying once, you have to go to System Settings -> Privacy & Security, scroll to the end and click Open Anyway for that app.