URL Rewrite for Category in Layered Navigation to External URL

In Magento 2 url rewrite module does not use Indexer to create rewrites. Rewrite is generated in process of save/import of product/categories/cms pages.

It turns out the Redirect directive in an .htaccess file won't match against the query string, only the request uri (the part before the question mark). To match against the query string you'll need to use the RewriteCond and RewriteRule directives. More info on this here: https://simonecarletti.com/blog/2009/01/apache-query-string-redirects/

This should work in the .htaccess

RewriteEngine On

RewriteCond %{QUERY_STRING} ^main_page=product_info&products_id=331$
RewriteRule ^index\.php$ http://gearx.local/catalogsearch/result?q=sku-331 [R=301,L]

If all the urls you need to redirect are in the same format with a numeric id then these two lines should take care of them all:

RewriteCond %{QUERY_STRING} ^main_page=product_info&products_id=([0-9]+)$
RewriteRule ^index\.php$ http://gearx.local/catalogsearch/result?q=sku-%1 [R=301,L]

I'd recommend redirecting to the actual url of the product though rather than a search results page.