O macOS Mojave is coming and, with it, the Safari 12. The new version of the Apple browser, as expected, will bring a series of novelties in the form of new features and support for recent technologies, but it will also be the first to abandon an important ally of many users: the old ones extenses browser, characterized by (not to be confused) the .safariextz file extension.
In case you got lost, remember: Apple introduced extensions for Safari in 2010, creating a gallery on its website where it was possible to download the files and install them in the browser. Less than a year ago, however, Ma started undermining the online repository and targeted Mac App Store developers, where extensions should be distributed / sold from there. Now, the old utilities, which did not make the trip to the store, can no longer be used.
All of this is detailed on the newly updated page What’s New in Safari (which is new in Safari), where Apple explains that, starting with version 12 of the browser, it will no longer support extensions not distributed by the Mac App Store even if they have subscriptions from registered developers and, in addition, display a warning about extensions using the canLoad event stating that that utility can be detrimental to the machine's performance; Ma recommends that developers replace the functionality with content blocking extensions.
Apple also has a page with a guide for developers to turn old Safari extensions into App Extensions (that is, extensions that can be distributed on the Mac App Store). In addition, for those who still want to install an old extension on Safari 12, developer George Garside has posted a solution tutorial on his website, which is a bit complicated, however, and should only be adopted by advanced developers or users.
It is good to note that the Safari extension gallery will continue to accept the submission of utilities until the end of 2018, but Apple recommends that developers switch to the new format instead to avoid problems and stay abreast of their latest extension technologies.