Send a 404 error via htaccess?

I think, 410 error better

RewriteRule ^directory/ - [L,R=410]

or my search:

RewriteCond %{HTTP_USER_AGENT} (googlebot|bingbot|Baiduspider) [NC]
RewriteRule .* - [R=410,L]

on russian wiki

404 Not Found [19] - the most common mistake when using the Internet, the main reason - an error in writing the address of a Web page. The server understood the request, but did not find the corresponding resource at the specified URL. If the server knows that there was a document at this address, then it is desirable for it to use code 410. Answer 404 can be used instead of 403 if you need to carefully hide certain resources from prying eyes. Appeared in HTTP/1.0.

Try:

RewriteRule ^directory/ - [L,R=404]

This redirects all requests for the folder "/directory/", they get a 404 response.

You could also for the time being change which page a user will see when confronted with a 403 error, but I wouldn't recommend doing this long-term.

.htaccess:

ErrorDocument 403 /your404pagehere.php