Symfony 4 softdelete with Doctrine ORM

I would guess that what you are missing is to enable the extension in the file config/packages/stof_doctrine_extensions.yaml that was added by the flex receipt.

It looks like, per default, it reads

stof_doctrine_extensions:
    default_locale: en_US

When, if you want to use soft deletable, you would need to activate it:

stof_doctrine_extensions:
    default_locale: en_US
    orm:
        default:
            softdeleteable: true