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).
- 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