How to activate Markdown in a local installation of WordPress 4.3?

Maybe you can try Markdown Editor plugin. It supports WordPress 4.0 - 5.3

Source code: https://github.com/terrylinooo/githuber-md

WordPress 4.3 have no markdown implementation, it is only a short list of shortcuts. The shortcuts active on default. It give no UI area to active/deactivated them. As small hint for updates posts. I seen often, that works only on new posts, no old drafts. But I haven't debug this.

Which Formatting Shortcuts

WordPress 4.3 came with a new feature called formatting shortcuts. It allows users to quickly add common text formatting without removing their hands from the keyboard and without writing any HTML.

• Using * or – will start an unordered list.
• Using 1. or 1) will start an ordered list.
• Using # will transform into h1. ## for h2, ### for h3 and so on.
• Using > will transform into blockquote.

Deactivate shortcuts

The only way to deactivate is a small piece of code, like in a simple custom plugin.

add_filter( 'tiny_mce_before_init', 'fb_disable_mce_wptextpattern' );
function fb_disable_mce_wptextpattern( $opt ) {

    if ( isset( $opt['plugins'] ) && $opt['plugins'] ) {
        $opt['plugins'] = explode( ',', $opt['plugins'] );
        $opt['plugins'] = array_diff( $opt['plugins'] , array( 'wptextpattern' ) );
        $opt['plugins'] = implode( ',', $opt['plugins'] );
    }

    return $opt;
}