The reading lists extension for Safari is a legacy .safariextz-style extension and is no longer available through official Apple channels as of the sunset of the Safari Extension Gallery in September 2019. It will require converting to a Safari App Extension if we wish to distribute it again through the app store.

https://developer.apple.com/documentation/safariservices/safari_app_extensions/converting_a_legacy_safari_extension_to_a_safari_app_extension

If possible, we'd likely want to understand usage of the existing tech and also understand plans for eventual web implementation.