The Debian GNOME team will be uploading GNOME Shell 49 to Unstable
"soon". It is currently available in experimental.

Many packaged GNOME Shell extensions compatible with 48 can be made
compatible with 49 by simply patching the metadata.json file to add
"49" to the list.

Or, it's possible that code changes will be necessary.
Upstream provides a migration guide at
https://gjs.guide/extensions/upgrading/gnome-shell-49.html

Thank you,
Jeremy Bícha