Is there a is_ function for sitemaps?

There is no is_sitemap() core function in WP < 6.8, but there is a ticket for it here.

The rewrite rules for the sitemap (src) use e.g. the sitemap query variable:

add_rewrite_rule( '^wp-sitemap\.xml$', 'index.php?sitemap=index', 'top' );
add_rewrite_rule(
    '^wp-sitemap-([a-z]+?)-([a-z\d_-]+?)-(\d+?)\.xml$',
    'index.php?sitemap=$matches[1]&sitemap-subtype=$matches[2]&paged=$matches[3]',
    'top'
);
add_rewrite_rule(
    '^wp-sitemap-([a-z]+?)-(\d+?)\.xml$',
    'index.php?sitemap=$matches[1]&paged=$matches[2]',
    'top'
);

The get_query_var( 'sitemap' ) is checked within the template_redirect hook to see if the sitemap should be rendered (among other things, like if the sitemap is enabled) (src).

An untested suggestion would be to check if get_query_var( 'sitemap' ) returns a truthy string according to the check made in core (src):

if ( ! function_exists( 'is_sitemap' ) ) {
    function is_sitemap() : bool {
        $sitemap = sanitize_text_field( get_query_var( 'sitemap' ) );
        return (bool) $sitemap;
    }
}

I also skipped my previous a-z+ check on $sitemap, as the core check does only care about if it’s boolish, after running it through sanitize_text_field().

We could also add to the function, a similar check for the XSL stylesheet with the sitemap-stylesheet query variable.

It might be safer to prefix the function’s name as well.

But in general it should be possible to use instead existing sitemap hooks, to target a specific sitemap problem, rather than creating a specific is_sitemap() function for it.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)