TYPO3 Core

the $view of the modifyView method is no longer a StandaloneView , it's an ViewInterface without a setTemplateAndFilename method. The render() method of the ViewInterface says "Optionally receives a template location" - but I think that path is meant inside of the extension and not to override the path here (I tried, it didn't work).

I took a look into the TYPO3\CMS\Backend\Controller\LoginController - the template paths of the modifyView method are hard-coded without an option to override:

$viewFactoryData = new ViewFactoryData(
    templateRootPaths: ['EXT:backend/Resources/Private/Templates'],
    partialRootPaths: ['EXT:backend/Resources/Private/Partials'],
    layoutRootPaths: ['EXT:backend/Resources/Private/Layouts'],
    request: $request,
);

It is possible to set a $templatePathAndFilename when creating a new ViewFactoryData - but not afterwards.

My code looks the same as the migration example in the link above.

Because we're continually decoupling Fluid from the Core, extensions need to start differentiating between View implementations as well. This will probably be a bit more seamless in the future, but for now we also use instanceof checks in the core, for example in ExtBase internals. So I'd suggest to use something like this in your case:

if ($view instanceof FluidViewAdapter) {
    $view->getRenderingContext()->getTemplatePaths()->setTemplatePathAndFilename($yourTemplate);
}

You can also use setTemplateRootPaths on that object and return the template name, that would probably be even cleaner.