Apple stopped supported for VPN networks that still use the not so secure PPTP protocol when they released macOS Sierra and iOS 10 (more info here)
In a nutshell this meant that the embedded system clients to connect to VPN in both the iPhone and the Mac don't have an option to connect to these networks anymore. The good news for the Mac is that you can still use clients like Shimo to connect to PPTP VPN.
As for iPhone and iOS 10 I think Apple hasn't officially said this, but some people in Forums (see 1, and 2) have confirmed that not only support was disabled for the system client, but Apple also blocked passthrough. What this means is that if you download a client to iOS or use Tethering (which is my main use), your PPTP will be blocked anyway regardless or what setup you have at your client source (e.g. A PC Laptop with Windows, a Mac with Parallels + Windows or with Shimo).
Does anyone know of a way to override this in iOS?
The only workaround I've found to be feasible is to create a server somewhere with L2TP/IPSec and from that server connect to the other VPN server using PPT, although this seems a bit complicated.