At WWDC 2020, in June 2020, Apple introduced Safari Web Extensions. These are once again allowing extension development with web technology through WebExtensions APIs, giving an alternative to Safari App Extensions. This was part of the Safari version 14 release.
On 2015-06-08, Apple revealed their new Apple Developer Program merging the three previous programs (iOS Developer Program, Mac Developer Program, and Safari Developer Program) into one. The merging means people who previously paid for 2 programs will end up only paying the single fee of 99 USD. 
Everyone who is a member of the free Safari Developer Program received an email stating they “can continue building Safari extensions and bring [their] creativity to other Apple platforms by joining the Apple Developer Program.” That means going from free to a yearly 99 USD payment.
This does not only effect people who want to distribute their extensions through Apple. Safari has always required a Safari Developer Certificate to install an extension through the Extension Builder. Users can no longer download the source of an extension and build it themselves, or even contribute to open-source extensions without joining the Apple Developer Program.
Apple states your “existing Safari Developer Program membership will remain active until July 8, 2015 and your Safari extensions will continue to work for existing users.” This presumably means Apple will force all certificates to be invalid after July 8.
- Is developing extensions for Safari free? at the Apple Developer Forums.
As of September 2019, the Safari Extensions Gallery was shutdown completely. App Extensions have to be downloaded from the Mac App Store instead. Presumably all the old extensions have now been lost. Per Jeff Johnson, developers only had about a week notice to the sudden shutdown.
- What’s New in Safari (archived) (accessed 2018-06-08).
- 2019-08-27 : Important Information Regarding the Safari Extensions Gallery (archived) (accessed 2019-09-08).
- 2019-09-04 : The Safari Extensions Gallery is no longer available (archived) (accessed 2019-09-08).
Resurrection as Web Extensions
You have to adapt it for WebKit, join Apple’s $99/year developer program, learn how to use Xcode and App Store Connect, and distribute via the Mac App Store.
Because distribution happens via the Mac App Store, all extensions must go through Apple’s review process. Thomas Reed tweeted on 2021-07-29 that Apple is not allowing extensions from submitters not classified as a "reputable company". Michael Tsai collected some of the follow-up on his blog.
- Safari 14 Release Notes (archived) (accessed 2021-01-16) notes: "Added Safari Web Extensions support for macOS.".
- 2020-06 : Meet Safari Web Extensions (archived) (accessed 2021-01-16).
- 2021-01-05 : How to convert existing web extensions for Safari (archived) (accessed 2021-01-16).
- 2021-01-15 : Safari 14 added WebExtensions support. So where are the extensions? (archived) (accessed 2021-01-16).
- 2021-07-29 : Tweet: "We had a free Safari app extension rejected by an App Store reviewer last night, because ..." (archived) (accessed 2021-07-30).
- via: 2021-07-30 : Safari Extension Rejected Because Developer Not “Reputable” (archived) (accessed 2021-07-30).
- https://mjtsai.com/blog/2019/09/26/safari-13-and-extensions/ - some choice responses to Safari 13 scrapping safariextz
- https://nakedsecurity.sophos.com/2019/09/24/apple-restricts-adblocking-extensions/ - with only apps available as extensions, adblocking suffered a severe hit