The change is that I suspect “the latest version” was always sufficient before. Now there is more need for older versions to remain available. So you basically need something where you can look up an extension, and then also select what version you want to download.
A package manager metaphor would work for this. mvnrepository.com is one for java/maven projects. archiva, nexus, there’s probably some open source package that can be adapted to this.
One thing that may be a complication is that it is encouraged for authorship of an extension to change when minor updates are made.
I think it’s still valid for the Inform IDE to continue using its public library, representing a subset of extensions that they have vetted and that fit certain other restrictions (I think they require creative commons licenses for being in that public library). But then there needs to be that one other “in-the-wild” place that has multiple versions of multiple extensions. github isn’t really sufficient for that because the collection of extensions will only reflect the most recent version of each.