I am not sure if I am quite getting your point here, but if we're talking about indexes, I've just implemented indexing using FoundationDB and writing to the index happens in the same transaction as the main data write (not really sure why it would ever be otherwise). Definitely not "+1 raft quorum check".
Calling FoundationDB "just a layer on top of TiKV" is… well… :-)
Calling FoundationDB "just a layer on top of TiKV" is… well… :-)