<header class="entry-header">
    <?php

    if (is_singular()) {
        the_title('<h1 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h1>');
    } elseif (is_front_page() && is_home()) {
        the_title('<h3 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h3>');
    } else {
        the_title('<h2 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h2>');
    }

    if ('post' === get_post_type()) {
        echo '<div class="entry-meta">';
        if (is_single() || is_archive() || is_home()) {
            keitaro_posted_on();
        }
        echo '</div><!-- .entry-meta -->';
    };

    ?>
</header>

<header class="entry-header">
    <?php

    if (is_sticky() && !is_single()):
        printf('<p class="sticky-title-sm">%s</p>', __('Must read', 'keitaro'));
    endif;

    if (is_singular()) :
        the_title('<h1 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h1>');
    elseif (is_front_page() && is_home()) :
        the_title('<h3 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h3>');
    elseif (is_sticky()) :
        the_title('<h2 class="entry-title"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h2>');
    else :
        the_title('<h2 class="entry-title h3"><a href="' . esc_url(get_permalink()) . '" rel="bookmark">', '</a></h2>');
    endif;

    if ('post' === get_post_type()) :
        echo '<div class="entry-meta">';
        if (is_single() || is_archive() || is_home()) :
            keitaro_posted_on();
        endif;
        echo '</div><!-- .entry-meta -->';
    endif;

    ?>
</header>

Add small title specifically for Sticky posts and restructure if-else blocks
Add small title specifically for Sticky posts and restructure if-else blocks
