Would you mind referencing the PR in question or detailing what elements of SE-0309 will be lacking in 5.7? Unfortunately SE-0309 only advertises #33767.
There is also #39492, and other peripheral changes. The changes mentioned in my previous reply are to fully implement the features for property and subscript requirements and is still a work in progress (the current state of the implementation covers only { get } properties and subscripts). This final step is more involved than I initially thought and it may or may not find its way to 5.7 depending on when the release branch will be cut off.