ESS and APP Search are related but not the same thing ...
ESS is the Elasticsearch Service which includes Elasticsearch and Elasticsearch App Search which is a Solution that "Sits On Top Of" Elasticsearch.
App Search is built to simplify the App Search experience and so not every feature of the underlying Elasticsearch is supported.
I do not think the same feature is available in App Search.
If you wanted to not index fields you will need to remove them from the incoming / source JSON
(I am double checking this, but pretty sure)