zackdesign

Turn your WordPress post categories into a search powerhouse! This plugin gives you a set of dropdown menus based on parent categories and their sub-categories. Users select one sub-category per parent, and the plugin filters posts matching the selected combination.
Available as:
<ul>
<li>Gutenberg Block — “Multi-Category Filter” in the block inserter (new in v4)</li>
<li>Classic Widget — “Multi-Category Selection” in Appearance > Widgets</li>
<li>Shortcode — <code>[mcsw]</code> on any page or post</li>
</ul>
Features:
<ul>
<li>AND / OR category filtering — find posts matching ALL or ANY selected categories</li>
<li>AJAX chained drilldown — selecting a parent dynamically loads its children</li>
<li>Pretty permalink URLs (<code>/categories/3,5/search_type/and/order/title/</code>)</li>
<li>Ordering by title or default</li>
<li>Configurable blank search behavior</li>
<li>Exclude specific categories</li>
<li>Lightweight — no jQuery, no external dependencies</li>
<li>Pagination support</li>
<li>Display form above results on category pages (optional)</li>
</ul>
Sample use case:
Real estate — set up parent categories like “Bedrooms”, “Bathrooms”, “Price Range” with sub-categories beneath them. Users filter listings by selecting from each dropdown.
Upgrading from v3.x:
<ul>
<li>PHP sessions have been removed entirely — the URL now carries all state, which means better compatibility with caching plugins, load balancers, and modern hosting</li>
<li>jQuery has been replaced with vanilla JavaScript</li>
<li>CSS classes have changed from <code>wpmcsw</code>/<code>wpmm</code> to BEM-style <code>mcsw-*</code> classes — update any custom CSS</li>
<li>The old <code>select-chain.js</code> in the plugin root has been replaced by <code>js/select-chain.js</code></li>
</ul>

Multiple Category Selection Widget

Filter posts by selecting multiple categories using dropdown menus. Available as a widget, block, or shortcode.

flynsarmy

Adds a widget that can displays subcategories of a given category (or top level). Has options to show/hide post counts, exclude categories, depth limits and/or empty subcategories.

Flynsarmy Subcategory List Widget

Adds a widget that can displays subcategories of a given category (or top level).

Multiple Category Selection Widget vs Flynsarmy Subcategory List Widget