The Required and Optional constraints are special ones meant to be used only inside the Collection one (to decide whether each field in the collection of fields is optional or required). They are not meant to be usable standalone.

Actually, this is how we use it:

$validator   = Validation::createValidator();
$constraints = new Constraints\Collection([
    'theField'  => [
        new Constraints\Required(),
        new Constraints\NotBlank(),
    ],
]);

Correct us if we are wrong..
The "break" was introduced from version 3.4.39 to latest one 3.4.40

Correct usage of the Collection constraints looks like that AFAICT:

$constraints = new Constraints\Collection([
    'theField' => new Constraints\Required([
        new Constraints\NotBlank(),
    ])
]);

Also note that fields for which you pass an array of constraints instead of an Optional or Required object get it wrapped in a Required automatically. So your code is equivalent to using a standard Required constraint inside a required field.

Thank you @stof for your explanation.

There is no RequiredValidator in any version of Symfony. They are only annotations used by the Collection.

see also #36723

see #36894