This change affects how clients will see the results of a {$meta: "sortKey"} projection or a {$meta: "sortKey"} aggregation expression. From a quick search, I didn't find any documentation for this feature, but if there are examples in the documentation, they should be upgraded to show the new format. A release note about the change might be warranted as well.
Previously, the result of {$meta: "sortKey"} would look like:
Unknown macro: {""}
With this change in place the same sort key will look like this:
["first component", "second component", ..., "last component"]
I don't think any drivers are relying on the format of the sort key, but if they are, they will likely need minor compatibility fixes. Message me or the server-query Slack channel, and we can work out the details. Thanks!