No. DAOS is a Domino server FEATURE, and is transparent to the client. Any client (older Notes versions, web, mobile, etc.) will be able to read and WRITE attachments to an NSF with DAOS ENABLED on the Domino server. In 8.5.1, a new feature was introduced to the Notes client that takes advantage of DAOS to eliminate transmitting attachment data that already exists in the destination DAOS server repository. Other CLIENTS will STILL work, but they can not take advantage of this, and will always transmit the attachment data. 

No. DAOS is a Domino server feature, and is transparent to the client. Any client (older Notes versions, web, mobile, etc.) will be able to read and write attachments to an NSF with DAOS enabled on the Domino server. In 8.5.1, a new feature was introduced to the Notes client that takes advantage of DAOS to eliminate transmitting attachment data that already exists in the destination DAOS server repository. Other clients will still work, but they can not take advantage of this, and will always transmit the attachment data.