Same syntax in 7.4 Documents
You tried what command are you still putting the . in the command?
In general accessing the . (system indices) directly is not generally a good idea / not a supported API use at your own risk.
Directly from the 7.4 docs...
Task management API
The task management API is new and should still be considered a beta feature. The API may change in ways that are not backwards compatible.
Returns information about the tasks currently executing in the cluster.
Request
GET /_tasks/<task_id>
GET /_tasks