Search posts by Tag

You can hook inside the result, so that you change the query and search about the tags.
The result is than a loop with all posts to this tag.

You can relized this inside the template, there are is the output for search or write a small plugin, that hook inside the query and change the query, if it your search; perhaps with your url param. For the plugin use the hook pre_get_posts.

see:

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