Hell, even Android ProtonVPN app can not only be used for free, but also without sign up:

If you prefer, you can use the app without a Proton Account. Simply tap Continue as guest. Doing this allows you to access all the free features available in the app. To access our premium features, you’ll need to sign in with a paid Proton VPN account.

https://protonvpn.com/support/best-android-vpn-app