So after I understood that the issue was just in vscode, I found that is a know issue of Intelephense due the fact that in older php versions, constants could not be declared in php traits

https://github.com/bmewburn/vscode-intelephense/issues/2024

the solution is to suppress the message using /** @disregard P1012 */ on top like this

MyTrait.php

<?php

namespace App;

class MyTrait {
    
    public function myTraitMethod() {
        /** @disregard P1012 */
        return self::MY_CONST;
    }
}