The dataplane API is an external product that itself uses the "stats socket". I personally don't have any experience with the dataplane API.

Changing certificates (https://docs.haproxy.org/2.6/management.html#9.3-add%20ssl%2...) and adding servers (https://docs.haproxy.org/2.6/management.html#9.3-add%20serve...) using the stats socket is both officially documented.

If you find the documentation insufficient or unclear, then this can be considered a documentation bug that I recommend filing here: https://github.com/haproxy/haproxy/issues