Doc bug:

External Processing Filter docs have boilerplate text

This extension is functional but has not had substantial production burn time, use only with this caveat.

but doesn't seem to be implemented at all (its just a commit of the proto):

https://github.com/envoyproxy/envoy/blob/master/source/extensions/filters/http/ext_proc/ext_proc.cc

suggest removing/hiding this filter until its usable or update the boilerplate text stating so.