It wasn't really a big deal. There was some pain and it became essential to have multiple versions of Ruby installed. But I haven't heard of anyone insisting on new code written for Ruby 1.8.
So languages can break backwards compatibility once in a while, and it doesn't always have to be the shitstorm it was for Python.
Early 3.x was bad, and that probably gave critical mass to the backlash. But there's no reason to care anymore about what early 3.x was like. Modern 3.x is a way better language.
AFAIK there was no breaking change in Ruby after 1.9, so anything that runs on 1.9 should run on any Ruby version up to the most current.
The biggest change in 1.9 was the support of different encoding and making UTF-8 the default encoding. Ruby did it differently than Python, so a string in 1.8 was a byte string, whereas a 1.9 string was a UTF-8 string. This off course broke many scripts.