How to apply CSS based on publishing date

You can compare the post date to the date of your choice and enqueue different styles depending on the result:

<?php

function my_enqueue_styles_depending_on_date() {

    global $post;

    $style_switch_date="20201230"; // the trigger date
    
    if ( is_single( $post->ID ) ) {

        $post_date = get_the_date( 'Ymd', $post->ID );

        if ( $post_date > $style_switch_date ) {
            wp_enqueue_style( 'css_for_newer_posts' );
        } else {
            wp_enqueue_style( 'css_for_older_posts' );
        }
    }
}

add_action('wp_enqueue_scripts', 'my_enqueue_styles_depending_on_date');

Make sure to wp_register_script() before you enqueue the style by it’s name (handle) or use the full wp_enqueue_style() syntax.