I use Formspree [0] for GitHub Pages hosted stuff, and it is dead simple. I've never been spammed, as it requires a human click ("I'm not a robot") for the email to send.

[0] https://formspree.io

Staticman[0] is great. Same principle where you only have to POST to an API but with user content stored in Github. There's a free hosted version and it's open source.

[0] https://staticman.net/

Edit: Disclaimer: the company I work for Sponsor their hosting.

A stupid (but working!) solution I came up with the other day. Submit forms from your static site to Telegram: https://telegram.me/submissionsbot