Hello,
You can create a Post Authentication Script (PAS) to configure that condition to connect to the VPN:
+ If device is Android or iOS > DENY the VPN Connection
For more info about PAS:
https://openvpn.net/vpn-server-resources/post-auth-programming-notes-and-examples/#pas-only-custom-authentication-script
You can also take the example of the MAC PAS and modify it to use the Platform instead of the MAC
Of course, this will need extra work and Python knowledge, but doable
Best Regards,
DynamoX