The difference between archive and full is in the garbage collection - a full node verifies all the state on the chain, but doesn't keep it all. An archive does. As such, the setting you're looking for isn't in the
--syncmode flag, but rather:
You can find that in the Geth command line docs, or in this Geth blog post in the section "Breaking Changes". There's also some discussion about
--gcmode archive in this StackExchange here.
Update: I just realized that I literally read the question wrong, and that the OP is looking for information on storing
ancient data in a different location than the "regular" chaindata while running an archive node.
The command line flag for storing
ancient data in a different location is indeed
--datadir.ancient. It was announced in the EF blog from the Geth team here. If memory serves me right, the node might not immediately move all data into
ancient, and it might be a more gradual process, perhaps explaining why the OP is not seeing an immediate change.
I figure I'll keep the old information about running an archive node up just in case someone else lands here looking for it.