Thanks for reaching out. Unfortunately, we have no plans to add `client_modified` to your folder metadata result, as it would require a rather substantial change to our filesystem infrastructure.
For API v1, the modified time of a folder is actually quite misleading. It reflects only when the folder was created and/or if the folder was moved/renamed. It does not change if the file content within the folder changes.
For that reason, we simply decided to stop returning modified time in API v2.