PHP recently did the same thing with PHP 7, which introduced type declarations for the scalar types. This meant we needed to reserve `int`, `float`, `string` and `bool`. To minimise backwards-compatibility breakage, we only reserved these as class/interface/trait names, and not in other contexts.

And it seems we were right to reduce the scope of it. PHP 7 prohibiting "String" as a class name ended up being a significant BC break affecting several projects. I imagine it might have been worse still if `string` was a reserved word.