How to change src to data-src for iframe inside WP content?

The code snippet in your question is a filter, but it isn’t built properly:

• A filter needs a function that takes in the thing to be filtered as the first parameter, and returns the new version
• Your function ignores the first parameter, breaking any changes that have already been made
• Your function doesn’t always return a value

So, lets start with the function:

function iframe_image_lazy_load($the_content) {
    $thecontent = get_the_content();
        if(!empty($thecontent)) {
            ... do modifications to iframes
            return $post->saveHTML();
        }
    }
add_filter('the_content', 'iframe_image_lazy_load', 15);

First, there are no iframes to modify, those happened on the_content filter, and got passed in as the parameter, but you ignored that and got the original.

As a result, the string you’re working with just contains OEmbed URLs, no iframes. Lets fix that:

function iframe_image_lazy_load($content) {
    if(!empty($content)) {
        ... do modifications to iframes
        return $post->saveHTML();
    }
}
add_filter('the_content', 'iframe_image_lazy_load', 15);

Notice I used variable passed to the function, instead of ignoring it and retrieving the original content. I also fixed the indentation levels, bad indentation is a great way to get bugs.

Next, the if statement is unnecessary, lets turn it into a guard and exit early:

function iframe_image_lazy_load($content) {
    if(empty($content)) {
        return $content;
    }
    ... do modifications to iframes
    return $post->saveHTML();
}
add_filter('the_content', 'iframe_image_lazy_load', 15);

OEmbed Filters

Thinking about these things in terms of HTML strings with iframes in them is a limiting and narrow way to view things. These are OEmbeds! So why not filter OEmbeds 🤔, we can even use browser native lazy loading! Why simulate lazy loading by undoing all the savings with JS to implement it?

Something like this:

function lazy_loaded_iframes( $code ) {
    return str_replace ( '<iframe ' , '<iframe loading="lazy"' , $code ); 
}
add_filter( 'embed_oembed_html', 'lazy_loaded_iframes' );