Gregory on Nostr: there was something similar called Gated that required "unknown senders" to donate to ...

there was something similar called Gated that required "unknown senders" to donate to charity in order to get into your inbox.

But, they closed as a company in September 2023. However, they open-sourced their code.

You can see an archived copy of their site at https://web.archive.org/web/20220420145949/https://www.gated.com/. Their current site is https://www.gated.com/. Their open-sourced code is on Github at https://github.com/get-gated/.