This was part of one of their A/B tests....you can still sort Google search results by date. Speaking of YouTube, YouTube still is not good enough features wise. I'm a software maximalist and I would like to be able to search comments on videos and more importantly being able to search your YT playlists. Also there is no search feature to search channels' videos on mobile unlike YT on desktop (web version). But the most tragic thing about YouTube is its main search engine; it just sucks, it gives you very few relevant results and most of the other results are recommended garbage or garbage that is not even remotely close to your search query.
Speaking of recommended videos and their corresponding algorithms. One huge thing for YouTube would be feature that would allow you to tweak your recommendation settings. For example filters like: Recommend me only videos in English or recommend me only Gaming and Music videos etc. There is a lot of room for improvement but Google is public profit driven company so they have to balance their expenses and chase revenue which in return means serving users "en masse" and not focusing exclusively on the quality of the product.