That function was introduced in Wordpress 5.5.0 so there you have it. Wordpress 5.5 is over 2.5 years old. You should really address the “reasons” that are blocking you from upgrading and upgrade Wordpress (and PHP).
Either the plugin or the Wordpress theme is going to break, and the theme is the one that doesn’t work on a current Wordpress. So to put it bluntly, this is a problem of the theme and not of the WP Discourse plugin.