This function ties the Stripe country dropdown into the 'active' countries set within Event Espresso -> General Settings. By default payment method forms display ALL countries, this forces the Stripe form to only display active countries.

ee_stripe_country_dropdown_filter.php

<?php //Please do not include the opening PHP tag if you already have one

function ee_stripe_country_dropdown_filter( $options, $form ) {
    if( $form instanceof EE_Billing_Attendee_Info_Form 
        && isset( $options[ 'name' ] ) 
        && $options[ 'name' ] === 'Stripe_Payment_Intent_and_Elements_Form' 
    ) {
		if(isset($options[ 'subsections' ]['country'])) {
			$options[ 'subsections' ]['country'] = 
				new EE_Country_Select_Input(
				null,
				array(
					EE_Country_Select_Input::OPTION_GET_KEY => EE_Country_Select_Input::OPTION_GET_ACTIVE,
					'html_label_text' => esc_html__('Country', 'event_espresso'),
					'required'        => true,
					'html_class'	  => 'ee-billing-qstn ee-billing-qstn-country', 
				)
			);
		}
    }
    return $options;
}
add_filter( 
    'FHEE__EE_Form_Section_Proper___construct__options_array', 
    'ee_stripe_country_dropdown_filter', 
    10, 
    2
);

ee_stripe_country_dropdown_filter_pm_specific.php

<?php //Please do not include the opening PHP tag if you already have one

function ee_stripe_country_dropdown_filter( $options, $form ) {
    if( $form instanceof EE_Billing_Attendee_Info_Form 
        && isset( $options[ 'name' ] ) 
        && $options[ 'name' ] === 'Stripe_Payment_Intent_and_Elements_Form' 
    ) {
		// Pull the payment methods related to this form and check if the admin name is 'Stripe limited'
		$payment_method = $form->payment_method();
		if($payment_method instanceof EE_Payment_Method) {
			if(!$payment_method->admin_name() === 'Stripe limited')	{
				return $options;
			}		
		}
		
		// We want to limit this payment methods country selection based on EE settings.
		if(isset($options[ 'subsections' ]['country'])) {
			$options[ 'subsections' ]['country'] = 
				new EE_Country_Select_Input(
				null,
				array(
					EE_Country_Select_Input::OPTION_GET_KEY => EE_Country_Select_Input::OPTION_GET_ACTIVE,
					'html_label_text' => esc_html__('Country', 'event_espresso'),
					'required'        => true,
					'html_class'	  => 'ee-billing-qstn ee-billing-qstn-country', 
				)
			);
		}
    }
    return $options;
}
add_filter( 
    'FHEE__EE_Form_Section_Proper___construct__options_array', 
    'ee_stripe_country_dropdown_filter', 
    10, 
    2
);