I can't speak for any other engineer anywhere else (where I'm sure they synchronously send out email because they haven't learned how to do it better), but there's not a chance github do this at their scale, and they're built on Rails. Rails gives you async mail by default and you just have to plug in a queue adapter for your worker processes to consume.

e.g. GitHub gets this request, queues the job in redis/zero MQ/SQS or whatever they're using, and another process dedicated to sending those emails (or jobs with that priority) does the rest of the work.

This is a massively common pattern in the Rails world and is as trivial to configure as your database connection.