The thing is, I first thought that it was not updated frequently simply because the server side of Signal is really simple (it's only role is to forward messages after all), so the code was very rarely touched.
However there seems to be people that have tried to run their own infrastructure that are not able to get some features to work.
Would it be even legal for them to run an updated version of the server without releasing it as FLOSS given the AGPL license?