Safari extensions add extra functionality to the browser. They work like mini-applications on Macs, are installed and behave like the applications themselves. If you no longer want to use an extension in Safari, see how you can uninstall it.
Mac users running Safari 12.0 or later (running macOS Mojave or later) will see the extensions appear as applications. In fact, the older Safari extension file is no longer supported.
Like the process of installing Safari extensions, deleting them is complicated. Safari extensions are taken individually applications. This means that to remove an extension, you must delete the accompanying application (after first disabling it).
You can manage extensions from the menu Preferences of the browser. Click the "Safari" button from the menu bar and select the "Preferences" button.
From here, go to the "Extensions" tab. You will now see a list of extensions on the left sidebar. To disable an extension, do click on the radio button next to it.
The extension will now disappear from the extension bar.
Now you can delete the extension. From the Extension Information section, click the "Uninstall" button.
You will now see an emerging message which states that the extension is part of the application and that in order to uninstall the extension, you must remove the application itself. Here, click the "Show in Finder" button.
Enter the username and password password of your Mac, and then click OK.
The extension will be deleted now. If you see a pop-up message stating that the application was not deleted because it was in use, you should return to the Safari Preferences menu and make sure that extension is disabled. Again, make sure all sub-extensions are as well off.
Once the extension is deleted (move to waste bin), you will see it disappear from the Safari extension bar and from Safari Preferences.
Source of information: howtogeek.com