Shard key value size limit is documented at https://docs.mongodb.com/manual/reference/limits/#Shard-Key-Size (and possibly elsewhere), which will need to be updated. This change should also be mentioned in the release notes.
Shard key values have a max size limit of 512 bytes which is enforced on mongos when writing new documents (e.g. for inserts) and when validating the split points when committing a chunk split.
Because of this, if a user refined the shard key of their collection to include a field all documents have large values for, they would be unable split their chunks because all possible split points would be over the limit. To avoid this scenario without scanning the documents in a collection before refining the collection's shard key and now that that limit on index keys has been lifted (PM-284), this limit should be removed.