GitHub just added this functionality to Pull Requests and Issues and it looks great.
Replying with the same response to Issues and Pull requests over and over can be tedious. Saved replies allow you to create a response to Issues and Pull requests and reuse it multiple times. Saved replies are available on all repositories starting today. This can save you a ton of time typing and posting the replies you use most frequently.
Thanks to @Einsteino my template manager plugin is to the proof-of-concept point. Itâs missing a translation, but it is functional and allows creating templates on the admin interface.