As the author of the DAG Versioning AIP, I can say that this work has been deferred post 2.0 mainly to support end-to-end DAG Versioning.
Originally, we (Airflow Core Committers) were planning to have a Webserver-only DAG Versioning i.e. to improve the visibility behaviour but not execution:
The scope of this AIP to make sure that the visibility behavior of
Airflow is correct, without changing the execution behaviour which
will continue to be based on the most recent version of the DAG.
This means it overcomes the issues where you can go back to an old version of the DAG, to view the shape of the DAG few months back and you can see the correct representation instead of "always-latest".
Currently, Airflow suffers from the issue where if you add/remove a task, it gets added/removed in all the previous DagRuns in the Webserver.
However, what we have decided is that we will accomplish Remote DAG Fetcher + DAG Versioning and enable versioning of DAG on the worker side, so a user will be able to run a DAG with the previous version too.
Currently, we don't have a date but mostly planning to do it around 2023/24.