We are done with NNBD transition for a few years now.
Dart 2.12 (released March 2021) introduced null-safety.
That started a 2 year transitionary period during which you could mix nullsafe (language versions 2.12 or above) and non-nullsafe (language versions below 2.12) code in one program.
Dart 3.0 (released May 2023) removed support for language versions prior to 2.12 - meaning that you can no longer opt out of null-safety.
