As of #24503, themes are able to indicate that they support having non-link blocks in existing menu areas by adding add_theme_support( 'block-nav-menus' ).

However, it may be the case that themes want to use blocks in one menu but not another. We should look at making the block-nav-menus setting more granular.

One approach might be to make it a per menu location setting instead of a per theme setting by updating register_nav_menu to accept a supports argument.

register_nav_menu(
	'primary',
	array(
		'description' => __( 'Desktop Horizontal Menu' ),
		'supports'    => array(
			'blocks' => true,
		),
	)
);

See #24503 (comment).