Deprecating it is a breaking change and would make migrating code more difficult. It's like how semicolons are allowed but rarely used in Ruby, Go, etc.

The style guide says to omit "new" [1], and our formatter can remove them for you [2], so the effect is that they are gone without the pain of a forced migration.

[1]: https://www.dartlang.org/guides/language/effective-dart/usag...

[2]: https://github.com/dart-lang/dart_style