The problem is not just an AGPL violation here, even though the license explicitly requires to show the code if you are providing a service on top of it. According to AGPL-3, if you are using the service you are the user. Good luck anyway submitting such a request to them at this point.
The actual problem is that #signal is no longer willing to publicly share the sources of their server platform, which is what #signalapp users criticized the most about others in the past, #telegram in particular.
@danielinux@Arcaik@mmu_man agreed. Even if they're not legally obligated to have a public repo up, *not* having one up in this day and age of "everyone and their dog can throw a Gitea instance on a small VPS" for a project like this is just a bad look.