$ heroku buildpacks:remove heroku/nodejs -a zundan-mastodon
$ heroku buildpacks:remove heroku/nodejs -a zundan-mastodon-streaming
well, actually, buildpacks have not really been set for production apps:
heroku buildpacks -a zundan-mastodon
=== zundan-mastodon Buildpack URL
heroku/ruby
$ heroku buildpacks -a zundan-mastodon-streaming
zundan-mastodon-streaming has no Buildpack URL set.