We're using TechFellow.Optimizely.AdvancedContentArea for display options and up until recently it has been possible to read ""AdvancedContentArea__SelectedDisplayOption" from the ViewComponentContext.ViewData. After updating from 12.19.1 to 12.21.0 this has stopped working.  

protected override IViewComponentResult InvokeComponent(WebHighlightBlock currentContent)
{
	var displayOption = ViewComponentContext.ViewData["AdvancedContentArea__SelectedDisplayOption"] as EPiServer.Web.DisplayOption;
	var vm = new HighlightBlockViewModel
	{
		HighlightList = currentContent.HighlightList,
		IgnoreBreakout = !_renderingHelperService.DisplayOptionIsFullWidth(displayOption?.Tag),
	};
	return View(vm);
}

I saw there was a bugfix in the most recent update for the viewbag in blocks being empty, but this seems to be unrelated.
https://world.optimizely.com/support/bug-list/bug/CMS-32835

TechFellow.Optimizely.AdvancedContentArea has not been updated, so the issue is not here.