I commend you all on pushing the business in a new direction and am thankful that the retired plugins are still available on GitHub!
Unfortunately I was using one of the plugins that was retired via composer and noticed that builds were failing due to the plugin no longer being available via the wpmudev.org composer repo.
Thoughts on publishing the retired plugins to https://packagist.org so they can still be easily incorporated into a composer based WordPress pipeline? It appears that each plugin would just need to add a composer.json file and then be submitted to https://packagist.org.
I suppose I could fork the plugins I am using, add the composer file there and publish to packagist under a new account, however if there are eventually any updates on those retired plugins (security patches, PRs from members, etc), I would need to pull in those upstream changes as needed which would be a lot more work than bumping a version number.
I am more than happy to help with this. Thank you!