I've seen breaking changes between Ruby 1.8.6 and 1.8.7, so I'm really suspicious of this claim. Start with which version of Ruby are breaking changes not to be expected until 3.0?
Ruby doesn't follow SemVer, so expecting no breaking changes between 1.8.6 and 1.8.7 wouldn't necessarily be correct. I don't remember what the policy was at the time.