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.
Gnome doesn’t really have an extension API and instead the extensions hook directly into Gnome Shell. This allows extensions to do basically anything, but each new Gnome release might break an extension (if the used code path is changed).