Today we allow for custom index names. This adds ambiguity as fields might be referred to either according to their index name or their field name. What should happen when several fields have the same index name or if the index name of a field is the same as the full name of another field?

My understanding is that custom index_names are not useful anymore now that we have copy_to in order to index several logical fields into a single physical one, so I'd like to deprecate custom index_names so that the index name would always be the same as the full name.

Relates to #4081, #8870