> If I push a repo to Heroku that includes submodules, presumably Heroku then fetches those submodules

You're missing the point. You may use submodules hosted on github with Heroku, but you don't use Heroku to host that repo. You're not going to `git submodule add git@heroku.com:project`. So for the sake of argument, if we pretend that git repo hosts do need to use `git@`, I don't see a single reason why Heroku would.

> And if you're going to suggest that it should only consult users who have access to the repo, for a public project that's everybody, which makes it functionally identical to git@

Now you're confusing two things. Do you want to clone a public module as a subrepo, or allow commit access? Public repos can be cloned without identification. If you want commit access, why would project-level not work?

> Sure it does. IdentityFile is explicitly allowed to be specified multiple times for a single host, and the files will be tried in turn

Again, missing the point. If you don't specify a different host, you'll always be identified and authenticated as the first key that matches, therefore you'll only use a single account. That's why you have to use different hosts.

> And no, your own anecdotes do not constitute proof that providers often have to deal with this.

If you're not going to believe anything I say, I got nothing. Otherwise, 2 things

- I opened an issue and the response was basically "Ooooo that explains some of those tickets". They specifically mentioned issues with vagrant. - I presented this at a local meetup and someone else had put themselves in this position.

> Your work is handing out a shared public/private keypair and encouraging people to set this up as a default identity in SSH?

No. As I said, it was meant to setup the vagrant box and then not be used. By default vagrant connects using an insecure keypair.