Why “Settings->Permalinks” creates .htaccess file on nginx server?

WordPress tries to detect the webserver’s type by peeking into the global $_SERVER['SERVER_SOFTWARE'] variable.

The Apache check is:

$is_apache = (strpos($_SERVER['SERVER_SOFTWARE'], 'Apache') !== false 
    || strpos($_SERVER['SERVER_SOFTWARE'], 'LiteSpeed') !== false);

and the Nginx check is:

$is_nginx = (strpos($_SERVER['SERVER_SOFTWARE'], 'nginx') !== false);

You should therefore first check out:

var_dump( $_SERVER['SERVER_SOFTWARE'] );

to see if it’s correct.

Here are some alternative workarounds:

It’s also possible to avoid the .htaccess file being created with:

add_filter( 'flush_rewrite_rules_hard', '__return_false' );

to avoid the save_mod_rewrite_rules() to run.

Within the save_mod_rewrite_rules() function, there’s a got_mod_rewrite() check before the .htaccess is updated. It’s also possible to use:

add_filter( 'got_rewrite', '__return_false' );

to avoid insert_with_markers() to run that writes the .htaccess file.