Because the IP‘s of the proxy/VPN you use is publicly known. So as soon as they receive a package from that IP they know the user is using such services.
You are not logged in. However you can subscribe from another Fediverse account, for example Lemmy or Mastodon. To do this, paste the following into the search field of your instance: !piracy@lemmy.dbzer0.com
⚓ Dedicated to the discussion of digital piracy, including ethical problems and legal advancements.
Because the IP‘s of the proxy/VPN you use is publicly known. So as soon as they receive a package from that IP they know the user is using such services.