Adding text shadow to slide caption

can you share the site you need help with, then we can help you with the required custom css you need to add to your site to achive that effect

This is the page
https://secularhumanism.org/
The slider is located on the homepage and is our only instance

Our current workaround is finding appropriate images, but sometimes we have to use certain images where a text-shadow could potentially be useful.

add the following code to your Customizer -> Custom CSS to achieve a basic black text shadow.

.wp-block-eedee-block-gutenslider .slide-content h2 {
text-shadow: 3px 3px 0 black;
}

read more about css text shadow here:
https://developer.mozilla.org/en-US/docs/Web/CSS/text-shadow?retiredLocale=de