By now it is probably no longer news to many: GNOME Shell moved from GJS’ own custom imports system to standard JavaScript modules (ESM).
Extensions that target older GNOME versions will not work in GNOME 45. Likewise, extensions that are adapted to work with GNOME 45 will not work in older versions.
You can still support more than one GNOME version, but you will have to upload different versions to extensions.gnome.org for pre- and post-45 support.
Please file bugs with your favorite extensions or have a friendly conversation with your extension writers so that we can help minimize the impact of this change. Ideally, you could help with the port and provide a pull or merge request to help maintainers.
I really, really hope Cosmic turns out to be a good DE, because Gnome does a lot of cool stuff that I really like, but the actual experience of using it is miserable for me. It always feels like it’s fighting against everything I want to do.
I’m glad Gnome exists, but we need an option that does some of the cool and unique things they do while also being less opinionated.