Debian sid ('unstable') isn't hugely behind, at least by my standards of software upgrading. Fish 2.0.0 ended up in there about 8 weeks after release, and I'd expect 2.1.0 should get there in less time (2.0.0 was a change of upstream, and the first release in 4 years, while 2.1.0 is a relatively routine update from a distro's perspective).