[Proposal] Expanding Flutter's deprecation policy

[Kate Lovett]

Hello everyone,

A little over two years ago, we started enforcing a deprecation policy in order to keep the framework clean and clear for everyone.

This policy brought with it the introduction of Flutter fix to help migrate deprecated code, along side our long practice of providing migration guides when making breaking changes.

Over the past few years we've only applied this policy within the flutter package. This allowed us to iterate on the process and gather feedback from users to provide as smooth a transition as possible. Given the feedback we have received, we would like to expand the policy now to include the flutter_test package.

I have opened https://github.com/flutter/flutter/issues/122323 for tracking this proposal. For now, I'd like to welcome any feedback in that issue. We would not be implementing this until the next round of deprecations are up for removal.

This typically occurs around once a quarter after a stable release; we just finished the latest batch announced in https://groups.google.com/g/flutter-announce/c/tgl6nT_Y-uo The next batch will not start until after the next stable release.

Thank you!