I resolved it as follows: • I closed the Xcode project. • I renamed the parent folder from, let’s say, “project” to “_project.” • I reopened the project, and that was it. Thanks to everyone for the help!

I just tried enabling "Allow unsigned extensions," but it's still not showing up. To rule out a coding error, I generated a new test project using the code from the original. The copy was visible, but the original was not. It seems as if the extension is still installed in Safari, I imagine. Best regards.