Of course. Nifty thing about open source means I can configure a system to allow or disallow anything. Each server operator can monitor their legit users traffic and find what they need to allow and dump the rest. Corporate VPN's will be using known values. "Free" VPN's can vary wildly but one need not support them if they choose not to. On some systems I only allow and MSS of 1460 and I also block TCP SYN packets with a TTL greater than 64 but that matches my user-base.