-1. If it has already been rendered once, there is no reason to set it to false again

I have the Problem, that i set a formview rendered but i doesnt rendered it.
And now i the form_widget rendered...

Well, why are you setting it as rendered without rendering it ?

Because i use After the Set rendered = True a form_widget...
And After a Few steps i Need the Not rendered Form...

Its a change which made Things better and which is compatible which the Last Versions....

I still don't understand why you need to set a form view as rendered without rendering it and then set it again as not rendered. Please explain your use case

Having a form template like...

a.html.twig

{% block content %}
    {% block form %}
        {{ form_widget(form) }}
    {% endblock form %}
{% endblock content %}

b.html.twig (render this)

{% extends "a.html.twig" %}

{% block form %}
    {% form.xyz.setRendered() %}
    {{ parent() }}
    {# ...... other things ........ #}
    {% form.xyz.setRendered(false) %}
    {{ form_widget(form.xyz) }}
{% endblock form %}

And i use a twig extension for that, too.

That's not a use case that we want to support in core as it looks really weird. There is probably a better way to achieve what you are trying to do... which we cannot really say with the small snippet of code your provided in the comment above.