How to remove trailing slashes from URL with .htaccess in localhost?

Since you already have the DirectorySlash Off directive, what you are probably seeing is a cached redirect.

Normally, mod_dir (DirectorySlash On) will append a slash when requesting a real directory (this is required to make the request valid) by issuing a 301 redirect to the slashed URL. 301 (permanent) redirects are naturally cached by the browser.

In order to still serve the DirectoryIndex then you need to now do this manually yourself and internally rewrite to the slashed URL, or explicitly to the index document (eg. index.php) if this is known. Otherwise you'll likely get a 403 Forbidden (unless mod_autoindex is configured to generate a directory listing).

So, something like the following:

DirectorySlash Off

RewriteCond %{REQUEST_FILENAME} -d
RewriteRule !/$ %{REQUEST_URI}/ [L]

The mod_rewrite directives state... for all URLs that don't end in a slash that do map to physical directories then internally rewrite and append a slash. This will then allow mod_dir to further rewrite the request and serve the DirectoryIndex (ie. index.php).

I assume all your internal links in the HTML do not include the trailing slash.

You might, however, need to issue a 301 redirect if the slashed URL is accessed directory - but you only really need to do this if these URLs have already been indexed by the search engines or are being linked to.