+1 with the following provisos:
* we need to update the breaking changes section of 3.0.0 release notes on the website (site/src/site/releasenotes/groovy-3.0.adoc) to include the cases listed in GROOVY-8561
* we need to update the "Array initializers" section of core-differences-java.adoc (we need to explain what cases remain - obviously if no array type is specified is one case - I haven't checked whether Java allows arrays of lambdas or something similar which we may not support - I don't believe so but haven't researched into it)
* we need to update the "Arrays" section of core-syntax.adoc (I think we need to indicate that square brackets is the idiomatic approach and also explicitly say that closure is the default interpretation of curly braces unless the array target type can be inferred from the context.)
I think it is probably worth adding the last two into the PR for any future reviewers. We should also update the "Lists" section of core-syntax.adoc to include information about Groovy's auto promotion of a single item into an array. I guess that isn't really part of this change but since that is where the breaking changes changes will be for the degenerate edge cases, I think we had better make that clearer.
I am happy to help with the wording in the doco changes (time permitting).