Unless you're on a self-hosted VPN (defeating the whole purpose), it's not especially hard to identify VPN connections. All of the common ones are known, and many use IP ranges and reverse lookups that clearly identify the VPN/seedbox provider.

It's a bit harder when you are connected to one that resolves to a residential-looking hostname. But again, unless it's truly unique (defeating the purpose), simply sorting users by IP will reveal almost all of them.

Some trackers used to do this to weed out people with multiple accounts. Some of the big ones still actively detect and block (or punish) anyone connecting to their website with a VPN (torrent traffic is still generally allowed, though)