What is the minimum version of RDP supported by Server 2012 RDS?

Microsoft confirmed that only RDP 6 and later is supported with Server 2012.

RDP 5.x clients cannot connect, except as follows:

RDP 5.x clients CAN connect if no license server is activated. In this case, Server 2012 RDS does not issue any license to the client, and allows the client to connect at a low encryption level, 512 bits.

However, when an RDS license server is activated, a higher level of encryption is required (2048 bits) and RDP 5.x cannot support this level of encryption. Therefore, the RDP 5.x client can no longer connect.

We paid for a support call and confirmed this with the tech.