(Firebase engineer here)
The old SDKs will continue to work! We worked extremely hard to make sure everything was backwards compatible with them. We most certainly do not want to break any existing customer apps. Check out the migration guides [1] to get your app updated to the new SDKs.
We understand migrating is not easy or always convenient. If you run into issues or something that used to work seems to be no longer work, please let us know. We will be giving you advanced warning if and when things get turned off.