Multisite, but wp_kses_allowed_html only for one subsite?

Well, you could do it this way:

if ( 101 === get_current_blog_id() ) {
  add_filter( 'wp_kses_allowed_html', allow_iframes_for_editor, 1 );
}

(Note: edited to use the get_current_blog_id() ; see https://developer.wordpress.org/reference/functions/get_current_blog_id/

But you should consider Tom’s comment as a warning against doing this.