Using a filetransfer on version 21.0 with the agents being on 21.0 and 12.x throws an error:
The TLS Gateway agent is required for filetransfers between TLS enabled agents (21.0 agents) and non-TLS enabled agents (12.x agents and a handful of 21.0 agents listed here).
Release : 12.3
Version 12.x agents in a filetransfer with 21.x agents must use the TLS Gateway to allow for transferring.
Version 12.x agents transferring with 12.x agents can connect to the system via a CP and not the TLS Gateway. The CP referred to here is the standard CP or ucsrvcp process that was in 12.x as well.