Yesterday afternoon I received an email from Apple Developer Relations titled "Important Information Regarding the Safari Extensions Gallery". According to the email, "the Safari Extensions Gallery will no longer be available in September 2019."
I haven't yet seen this email mentioned in the news media. I know it wasn't sent to all Apple developers, so I suspect that I received it because I'm the developer of a Safari extension. However, my extension StopTheMadness is in the Mac App Store rather than the Safari Extensions Gallery, so it's unaffected by this announcement. StopTheMadness is compatible with Safari 13 and the forthcoming macOS 10.15 Catalina.
Here's the text of the email:
As a reminder, the Safari Extensions Gallery will no longer be available in September 2019. Legacy Safari Extensions (.safariextz files) are not supported in Safari 13 on macOS Catalina, macOS Mojave, or macOS High Sierra. Users on macOS High Sierra or later can easily find extensions on the Mac App Store by choosing Safari Extensions from the Safari menu.
To make sure users can take advantage of your extensions on the latest versions of Safari on macOS High Sierra or later, we recommend that you convert your Legacy Safari Extensions (.safariextz files) built with Safari Extension Builder to the Safari App Extensions format and either submit them to the Mac App Store or notarize them for distribution outside the Mac App Store.
And here's a screenshot: