[POLICY] Release Symfony 5.4 with which Drupal branch and how much support will it get?

We want to release Drupal 10 with Symfony 6. As Drupal 9 is released with Symfony 4, we are skipping Symfony 5. The problem for Drupal is that Symfony has added deprecations to the Symfony 5 branch and those deprecations will be removed from Symfony 6. Some Drupal 9 sites will be using functionality from Symfony that will be removed in Symfony 6 and therefor will the same functionality also be removed from Drupal 10. We would like to give Drupal 9 sites the possibility to test with Symfony 5 for those deprecations. Having deprecation warning messages makes migration to a new version a lot easier then being confronted by hard breaks for functionality that has been removed. The decision has been made to create such a "test" branch with Symfony 5.4. The question now is how are we going to do that exactly. A couple of choices have to be made:

1. Which branch we shall use for the Symfony 5.4 test branch? We have the 9.3, 9.4 and the 10.0 branch to choose from. The 9.3 branch has the advantage that it can be release in november 2021 and allows sites to test before well before the release of D10. With the branches 9.4 or 10.0 we shall have to wait for march 2022 when we can do beta release. A stable release will be much later. The 9.4 branch with Symfony 5.4 will be a Drupal 9 site with all the BC layers still there. The same with the 10.0 branch will have all those BC layers removed.
2. The next problem is if the release will be just a single snapshot in time or will the branch with Symfony 5.4 get updates just like the regular branch?
3. Is the "test" branch just the same as the regular one only with Symfony 5.4 or do we need to do more? Deprecation message suppressions need to be removed for the "test" branch. Is this enough or do we need to do more?