Methods script() and stylesheet() are marked inside docblock with:

@ deprecated 4.0 The (file, framework, relative, pathOnly, detectBrowser, detectDebug) method signature is deprecated, use (file, options, attributes) instead.

So, I think the concerning parts will be removed finally in Joomla 4.

FWIW the proper method to add stylesheets and script ARE and will remain for Joomla 4 the: HTMLHelper::stylesheet and HTMLHelper::script (NOT $document->addStyleSheet)

What was deprecated and will be removed is the option to pass the parameters as comma separated. Now it's mandatory to pass them using an array, eg:
OLD WAY

HTMLHelper::_('script', 'some/path/to/script.js', true, true, false, true);

NEW WAY

HTMLHelper::_('script', 'some/path/to/script.js', array('relative' => true, 'version' => 'auto', 'pathOnly' => false, 'debug' => true);

PLEASE DONT USE $document->addStyleSheet AS YOUR ASSETS WILL NOT BBE OVERRIDABLE!!!

I get your point dgrammatiko

But then I recommend to change in HTMLHelper.php at the beginning of public static function stylesheet() from currently
if (!is_array($attribs))
to
if (!is_array($attribs) || !is_array($options))
since the css wont get included with these parameters anyway.

But this way the notices are gone and eventually a deprecated message is logged

@FoTo50 please feel free to submit a PR with that code

Closed see #21734