Alternatives of Heroku Free Plan

Not free, since you still need a Kubernetes somewhere.

But at least 100% open-source and selfhostable.

It covers most of Herokus features.

https://docs.kubero.dev/comparison-heroku/

Heroku's PostgreSQL is still free. As for dynos, the lowest tier is $5/mo for multiple apps (not production traffic of course, maybe for MVP), with custom domain support.

I'll vouch for dokku as a replacement. That has been such a great find for running relatively small projects on a cheap VPS.