Splunk configuration parameter Message_Key missing

[demian711]

Hello, We recently noticed that in version 1.9 of fluent-bit, configuration parameter Message_Key is missing for Splunk.
Looking documentation, seems that it was deprecated in 1.8, but there is no reference at all for this deprecation.
Even more, its only noticed because it disappears from an example and that's it

[error] [config] splunk: unknown configuration property 'Message_Key'. The following properties are allowed: compress, http_user, http_passwd, http_buffer_size, http_debug_bad_request, event_key, event_host, event_source, event_sourcetype, event_sourcetype_key, event_index, event_index_key, event_field, splunk_token, splunk_send_raw, and channel.

Is there any parameter to replace Message_key ?

many thanks