Extracting fields based on eventtype?

It is currently possible to setup field extractions based on an eventtype definition, but it sounds like this may not always be supported. I've been using this feature since Splunk 3.3 or 3.4 (when we first started using splunk). But based on some discussions with the engineers, it sounds like this feature may go away or be depreciated and it sounds like it's currently already a highly-discouraged feature.

I recently notice the following message in my info.csv in the job dispatch folder. (Seem's like it's a "DEBUG" message which is probably why I haven't seen it from the search UI)

Message:

Extracting fields based on eventtype is not supported during the main search. Please see splunk documentation for more information.

However I'm unable to find the related documentation. Anyone know the official answer to whether or not this feature is truly going away, and how soon? Would the message be more accurately stated: "searching for extracted fields based on eventtype is not supported during the main search"? Or is there some other meaning here?

I get that technically field extractions based on eventtypes is a complex and potentially confusing feature. I have many different types of events with unique field extractions for a single source/sourcetype; so I'm not sure what the recommendation is on how to replace my existing eventtype-based field extractions....