Add obsolete lifecycle flag to info file.

I think you misunderstood the timeline.
Config Filter will most likely remain supported until Drupal 11 LTS becomes unsupported. So we probably have at least three years to go.
I reserve the right to change this decision and end support earlier if a change in core makes it a hassle to keep it supported. And if there is a need (and annotation based plugins survive into Drupal 12) extend the support.