Does activating text forwarding between Apple devices forward old messages? If I turn text message forwarding on my iPhone to forward to Mac, how far back does it forward? In other words, does it only send to the Mac what was sent/recieved via text from the date the forwarding was turned on? Or does it download old texts deleted from iPhone as well?
Text message forwarding only applies to messages sent and received when the option is active. This is not retrospective and does not copy prior messages over.

Apple keep delaying the introduction of Messages in the Cloud which will sync all message history. At the time of writing, this functionality is not available.