Google used to group the options from the search sidebar so you can quickly find the filters related to location, visited pages and the different ways to present search results. All these groups have been merged and Google only kept the group that lets you restrict the results to recent web pages. Google also removed the "related searches" view which showed a list of related searches at the top of the page and allowed you to compare the results for different searches.
"Based on your search and the Google property you've selected, you'll see a customized list of filtering tools. We'll automatically show you the tools that seem most relevant to your search, so you won't always see all of these tools," explains Google. To display all the filters, click "more search tools".
Here's a screenshot that shows some of the old sections:
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