Based on my understanding, the old SignalR repository is dead: there were no commits for a year (ironically, the guys who previously committed to it, now commit to Node,js), dozens of issues without response (we experienced a bug and had to downgrade to a version that's two years old).
There's a SignalR-Server repository which is a part of ASP.NET. It was supposed to be released with ASP.NET 5 (Q1 2016) but according to roadmap, it will be released later (Q3 2016). So I guess this is the repository where the actual SignalR development is going on. But the appropriate client repository is empty. So should I use the client from the old repository and a server from the new one? That's extremely confusing.
A new version of SignalR has been released as a part of .Net Core 2.1 (30.05.2018)
Id'say alive and kicking, https://github.com/SignalR/SignalR/pulse I think you may have been looking at commits to the master branch only.
There will be a version 3 of signalR included in ASP.NET 5. The development team aim for a release by mid-2016. See this roadmap
Update #1 (April 2018):
mid-year 2018
(see ASP.NET Blog)Update #2 (June 2018)