People have come to expect that minor versions are interchangeable (modulo fixes for clear regressions and changes made in reaction to important outside events like major releases of Java).
It's also unavoidable.
In theory, either the people doing the minor version releases need to worry about it, make conservative changes, thoroughly test those changes against a large amount of code, etc. - or the people consuming the minor version releases need to worry about it and do much of the same thing. Either way, build tools that can handle compiler versioning is a win.
In practice, both the people doing the minor version releases and the the larger codebases consuming those minor version releases need to worry about it. "fixes for clear regressions and changes made in reaction to important outside events" is already a giant caveat that will lead to breakage at times, and even the best teams will occasionally fail to meet expectations.
All I'm saying is that the situation is bad as-is, and it would be good to not make it any worse.
It's unusual but not unique, see Python, Linux kernel, Postgres.
They are not, at least in Scala. See 2.12.x.