Replacement for CoRD (Microsoft RDP )
I've found, and am now loving, Royal TSX. Not only does it do exactly what I loved CoRD for - multiple RDP connections in a single window - it has a plugin system that'll connect to VNC, TeamViewer, web pages etc, and more in the future. The RDP plugin is based on FreeRDP, and seems to support the latest RDP protocol enhancements.

Some of the functionality requires the companion server product, and it'll only do 10 connections in "free" mode ($29 for unlimited connections). But given that it's a stable, feature-rich client that is being actively developed, I can recommend it highly.