Apple - Connection L2TP via IPSec to VPN as PPTP VPN no longer supported on macOS Sierra
The FlowVPN client is free (and works with any provider).
It supports PPTP and OpenVPN on macOS Sierra:
https://www.flowvpn.com/download-mac/
Just overwrite the FlowVPN server address with any server and it connects.
Not open source and not cheap, but Shimo is a way to get back PPTP connections. Is 54 USD though. Be aware though:
Does not support PPTP/L2TP on macOS Catalina