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.
It would make more sense to specify something like API versions, not software versions, and flag on the client when it changes without the addon being updated (giving you a choice to run it with a warning or not). That is, unless the version update is specifically flagged as breaking compatibility, in which case it would just warn and not offer to run it anyway until it’s been updated.
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).