If iCloud Advanced Data Protection is enabled, is there still a need to use the encryption key in DT to sync over iCloud between devices?
This setting does not affect iCloud (CloudKit) and therefore the sync’s encryption is still recommended. This is also compatible to all kind of versions of macOS & DEVONthink and not limited to the latest ones.
In addition to @cgrunenberg’s comments, why would you not want to use one? I’m not saying you should automatically be distrustful of Apple’s intentions, but this is your data on their servers.
Would you just hand me your wallet to hold onto?
Or would you lock it up in a lockbox first?
Encryption takes no appreciable time to apply when syncing and it just ensures your data is inaccessible to any prying eyes.
(And yes, this logic and analogy also applies to Dropbox, CloudMe, etc.)