Yes, VPNs aren't allowed. You risk being banned from the servers if you use one. You can be banned even if you've never cheated just because someone has done that in the past under that same IP. So in other words, you can be banned simply for having VPN. Therefore it is prohibited. Simple.
The official statement is that you won't be banned for proxy IP if your account is "old enough" but who knows what "old enough" actually means.
1+ year, 10 years?
Take the risk if you want, be aware that it is possible to get banned so it's against the rules.