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.
