Just moved off my 8 years old project google-webfonts-helper https://github.com/majodev/google-webfonts-helper from their free tier to my own private infra and replaced the current dyno with a 301 handler: https://github.com/kenmickles/heroku-redirect
AFAIK sadly Heroku does not provide some other _free_ permanent redirect option for their *.herokuapp.com sub-domains without actually running a dyno there.