Don't think Ruby is declining anymore tbh, neither is PHP. They were once hugely popular and then shrank back to moderate usage; any data to back up that Ruby is losing usage over the last 3-5 years?
Does that show it's declining? It's proportion of pull requests, not absolute pull requests. Ruby could still be growing if the whole system is growing.
Unlike Java/C#, I would say github is a good indicator for Ruby.