Description
Due to its broken behavior, PHP is phasing out the Serializable interface currently.

See:

• https://wiki.php.net/rfc/custom_object_serialization
• https://wiki.php.net/rfc/phase_out_serializable

The replacement are the magic methods __serialize() and __unserialize() that have been introduced with PHP 7.4. As long as our implementations of Serializable also implement those methods, PHP 8.1 won't throw deprecation notices at us. As far as I can tell, we're safe here.

For Symfony 6, we decided to not provide support for PHP 7 anymore. This also means that we don't need Serializable as a BC layer for PHP 7.3 and below anymore. Because of that, I'd like to propose to remove that interface from all Symfony classes in 6.0.

Affected by this change:

• Mime: RawMessage (the implementation of the interface is finalized and flagged @internal)
• Routing: CompiledRoute and Route both implement the interface with @internal and final.
• Security Core:
  • TokenInterface extends Serializable but also requires __serialize() and __unserialize() to be implemented.
  • AbstractToken provides an implementation that is again @internal and final.
  • NullToken implements Serializable without @internal and final.

Question now is: Do we need any additional deprecation layer for 5.4 or would it be okay to simply unannounce the interface and remove the implementations?