Regarding this method's deprecation:
ReflectionType::__toString() was first deprecated in PHP 7.1.0 alpha1.
The deprecation notice was removed in PHP 7.1.0 RC3, before being reinstated in PHP 7.4.0 alpha1.
Starting with PHP 7.1.0 beta 3, ReflectionParameter::getType() & ReflectionFunctionAbstract::getReturnType() return instances of ReflectionNamedType instead of ReflectionType.
The ReflectionNamedType class inherits from ReflectionType but provides an additional getName() method which can be used to retrieve the type hint.
Lastly, PHP 8.0.0 alpha1 introduced the concept of union types (see https://wiki.php.net/rfc/union_types_v2). As a result, ReflectionParameter::getType() & ReflectionFunctionAbstract::getReturnType() will now return either instances of ReflectionNamedType or ReflectionUnionType depending on the type hint, both of which are descendants of ReflectionType.