It's fine to break backwards compatibility when introducing an entirely new feature across a major version, actually. What's even the point of a major version bump?
Not everyone uses semver, although in any case Java "major" versions are actually a continuation of what was called the 1.x line -- they gave up on writing out the "1." a couple of decades ago. They're semver minor release, not semver major.
Plus, prepending the string with `STR.` changes the syntax entirely. Those strings are still invalid if you don't prepend `STR.` (which is another thing I hate, but not quite as much).
In C#, the string `"""Hello world"` is invalid, but `@"""Hello world"` is, because double quotes are valid with verbatim string literals.
There was a nice long deprecation period between those versions, where everyone using it got a warning. And then, when it was finally removed, everyone using it got an error, and they were forced to change their code. By contrast, a string literal could silently change meaning if it used syntax that was previously valid in strings. This is the exact same reason JS chose to require backticks to delimit interpolable strings - because otherwise existing string literals could silently change meaning.
I hadn't caught that this was required, but then it'd lock them out of removing that requirement. Plus it'd complicate the parser, to have there be two different kinds of string literals syntactically.
